Mom's Rhubarb-Apple Crisp

INGREDIENTS

Cooking spray

12 oz

fresh or frozen rhubarb, thawed, cut into 1/2-in.-thick slices

1 1/4 lb

Golden Delicious or Honeycrisp apples, peeled, cored, and diced

2/3 cup

granulated sugar

2 1/10 oz

all-purpose flour (about 1/2 cup), divided

1 tsp

vanilla extract

1/4 tsp

ground cinnamon

2/3 cup

old-fashioned rolled oats

1/2 cup

light brown sugar (not packed)

3 tbsp

unsalted butter, softened and cut into small pieces