Preheat oven to 350 F / 177 C. Line a 9x9 baking dish (an 8x8 will work too) with a piece of parchment paper or foil (this will help you to remove the bars after cooking) and spray it with nonstick cooking spray.
In the bowl of a food processor, pulse pecans and oats until finely ground. Add flour, both sugars and salt and pulse a couple times to combine.
Slice butter into 12 pieces and add to food processor. Pulse just until the butter is broken up into small pea-sized pieces. Transfer 1 cup of the mixture to a bowl for the streusel topping and place the bowl in the fridge.
To the remaining dough in the food processor, add egg, vanilla and baking soda. Blend until the egg is fully incorporated. Pour the dough into prepared baking pan. Wet your hands with tap water (this will prevent the dough from sticking) and press it evenly into the pan.
Bake crust until starting to turn golden brown on top, 10 to 12 minutes.
While the crust bakes, slice strawberries. If you think the slice berries could benefit from a bit of extra sweetness, toss them with 1 Tbsp of granulated sugar. (Fresh picked berries are usually quite sweet, but many store-bought varieties are much better with a tiny bit of extra sugar.)
Top the pre-baked crust with sliced strawberries and sprinkle the refrigerated streusel over top.
Bake until the strawberries are bubbly and jam-like and the streusel is golden brown, 45 to 50 minutes more.
Let cool to room temperature before slicing.
Bars will keep in the refrigerator for 3 to 4 days.