Preheat oven to 350 F. Line an 8x8 baking pan with parchment paper.
For The Topping
Combine all topping ingredients, except the butter, in a large bowl and mix together. Add the butter and work until all is crumbly and somewhat wet looking. We don’t want any dry spots in the mix. EXPERT TIP: You can use a pastry cutter or two butter knives, scissor style, to mix the crumble. I find it easiest to just use my fingers to combine the ingredients.
Take 2/3 of the bowl contents (approximately 3 scant cups) and press into a parchment-lined 8x8 baking dish. Pat it down firmly with the palm of your hand or the back of a measuring cup.
For The Filling
Toss prepared strawberries and rhubarb in a large bowl with lemon juice. Sprinkle sugar and cornstarch over the fruit, tossing to coat completely. Spoon fruit mixture over the crust.
Top with remaining 1/3 of crumb topping (approximately 1 cup) and bake for 35-45 minutes. The filling should be bubbling, and the topping should be set.
Cool on a wire rack. Remove from baking dish using the parchment paper and slice into squares or rectangles to serve.
Mix 1/2 cup powdered sugar with 1 teaspoon of milk. Add another teaspoon of milk if needed to reach drizzling consistency. Drizzle over the top of crumble bars before serving.
You can store your crumble bars in an air-tight container in the refrigerator for up to 4 days, though the longer they sit the softer the crumble may get. You can also keep these at room temperature for a few hours and they will be just fine!