Preheat oven to 350°F (175°C). Take out a 8"x8" baking dish and line it with parchment paper. Spray lightly with non-stick cooking spray.
In a large bowl, mix together the ingredients for the apple filling. In a separate bowl, stir together the ingredients for the pecan-oat topping. The texture should be slightly dry and crumbly. If it's too wet, add additional rolled oats and chopped pecans. If it's too dry, add a small amount of milk to thin it out.
Add your apple filling to the baking dish and spread evenly.
Sprinkle the topping evenly over the apples, breaking it into small clumps so it crisps rather than steams.
Bake for 35–45 minutes, until the topping is golden brown.
Let rest 10 minutes before serving. Top individual portions with Greek yogurt if desired.