The night before slow cooking, toss the vegetables with the olive oil and bake on a large baking sheet in a 375°F oven for about 15 minutes, or until the vegetables have begun to brown and soften. Cool to room temperature, then refrigerate overnight.

Grease a 6-quart slow cooker with cooking spray. Add the roasted vegetables to the prepared insert. In a large mixing bowl, whisk together the eggs, cheese, milk, salt, and pepper. Pour this mixture evenly on top of the vegetables. Cover and cook on low for 5-6 hours or on high for about 3 hours. The frittata is done when it has browned on top, the eggs have set, and it has begun to pull away from the edges.