Preheat your oven to 375°F. This takes about 10 minutes. While it heats, lightly butter a 9x13-inch glass or metal baking dish (about 2 minutes). You should smell a faint butter aroma when ready.

Wash and hull 2 pounds of fresh strawberries, then cut them in half. This takes about 10 minutes. Wash 1 pound of fresh rhubarb and cut into 1-inch pieces with a sharp knife (about 8 minutes). The rhubarb should look like small pink or red sticks.

Place the strawberries and rhubarb pieces in a large bowl (about 2 minutes). Toss gently to combine.

In a separate small bowl, whisk together 3/4 cup granulated sugar, 1/4 cup light brown sugar, 2 tablespoons all-purpose flour, 1 tablespoon cornstarch, 1 teaspoon vanilla extract, 1/2 teaspoon ground cinnamon, and 1/4 teaspoon salt (about 3 minutes). The mixture should look like a fine, spiced sugar blend.

Pour this sugar mixture over the fruit in the large bowl. Using a rubber spatula or large spoon, gently toss the fruit and sugar together, making sure the sugar coats all the strawberries and rhubarb (about 3 minutes). The fruit should glisten. Let this sit undisturbed for 5 minutes while you prepare the topping—a little liquid will start to release from the fruit, which is perfect.

Checkpoint: You are ready to move on when the fruit looks glossy and a small amount of juice has pooled at the bottom of the bowl.

Pour the entire fruit mixture (including all juices) into your prepared baking dish, spreading it in an even layer with a spatula (about 2 minutes). The filling should reach about halfway up the sides of the dish.

In a medium bowl, combine 1 cup old-fashioned rolled oats, 1 cup all-purpose flour, 1/2 cup chopped pecans, 1/4 cup sliced almonds, 1/3 cup light brown sugar, 1/4 teaspoon salt, and 1/4 teaspoon ground cinnamon (about 3 minutes). Stir with a fork or whisk until evenly mixed. The mixture should look like sand with visible nut pieces.

Cut 3/4 cup cold unsalted butter into small cubes (about 5 minutes). The butter should feel cold to the touch. Add the butter cubes to the oat mixture all at once (about 1 minute).

Using your fingertips, rub and pinch the cold butter into the oat mixture for 5–7 minutes. Work quickly so the butter stays cold. When done, the mixture should resemble coarse breadcrumbs with pea-sized clumps of butter visible throughout. If it's too fine and powdery, the topping won't be crispy.

Checkpoint: You are ready to move on when you can see small clumps (about the size of peas) throughout the mixture and it doesn't feel greasy—just crumbly.

Scatter the oat topping evenly over the strawberry-rhubarb filling (about 3 minutes). Use all of the topping. Gently press down on the topping with your fingers so it forms small clumps and adheres slightly to the fruit below—but don't pack it down too firmly or it will be dense rather than crispy.

Place the baking dish on the center rack of your preheated 375°F oven. Bake for 40–45 minutes. Set a timer for 40 minutes as a checkpoint.

The crisp is done when the oat topping is deep golden brown (not pale), and you see fruit juice bubbling up around the edges of the topping. The filling should smell intensely fruity and slightly caramelized.

Checkpoint: You are ready to move on when the topping is golden brown and the fruit juices are visibly bubbling at the sides (you may see them peeking out from under the topping).

Remove the crisp from the oven using oven mitts (about 1 minute). The dish will be very hot—do not touch the sides or bottom.

Let the crisp cool on a heat-safe countertop or trivet for 10–15 minutes before serving. This allows the filling to set slightly so it doesn't run all over the plate. The topping will crisp up as it cools.

Serve warm (not piping hot, and not cold) with a generous dollop of whipped cream or a scoop of vanilla ice cream. The contrast of warm crisp and cold ice cream is essential to this dessert.