Preheat your oven to 375°F and grab a 9x13-inch baking dish. In a large bowl, combine the sliced strawberries, chopped rhubarb, 1 cup granulated sugar, 1/2 cup brown sugar, 2 tbsp lemon juice, vanilla powder, and 1/4 tsp salt. Stir gently until everything coats with the sugar mixture — don't mash it up.

Pour the fruit mixture into your prepared baking dish and spread it evenly. The filling will look kinda liquidy right now, but that's exactly what you want — trust me on this. Let it sit while you make the topping.

In another bowl, mix together 1 cup rolled oats, 1/2 cup all-purpose flour, 1/3 cup chopped walnuts, 1 tsp ground cinnamon, and 1/4 tsp salt. These dry ingredients are the foundation of your crispy topping, so don't skip mixing them first.

Pour 1/2 cup melted butter over the oat mixture and stir until everything looks like crumbly sand. This is the step that creates those golden crispy pieces everyone loves. If it seems dry, add a splash more melted butter — you want it moist but not soggy.

Sprinkle the oat topping evenly over your fruit filling, making sure to cover all the strawberry rhubarb crisp filling. Don't pack it down — let it stay loose and chunky because that's what creates the crispy texture. You'll hear it sizzle when it hits the hot filling.

Bake for 35-40 minutes until the topping turns golden brown and you can see the fruit bubbling up around the edges. My oven runs hot, so I check mine around minute 35 — don't panic if the edges bubble more than the center. That's completely normal and actually means it's cooking perfectly.

Remove from the oven and let it cool for about 10 minutes before serving. I know it's tempting to dig in right away, but this resting time lets the filling set slightly so it's not watery when you scoop it.