Preheat your oven to 350°F and lightly spray a 2-quart baking dish with cooking oil spray.
Add the broccoli florets to a steaming basket and place it over a pot with boiling water. Cover and steam until just tender, then transfer the broccoli to the prepared casserole.
Meanwhile, heat oil in a large pan over medium heat. Add onions and sauté until translucent, 3-4 minutes. Stir in the garlic, mushrooms and smoked paprika and continue to cook, stirring occasionally for 5-6 minutes.
Meanwhile, in a small pot over medium heat whisk milk, cornstarch, salt, and pepper, until starts to thicken, 2-3 minutes.
Transfer the onion mushroom mix to the casserole, and pour over the milk sauce. Clean the pan with a paper towel, and lightly spray with cooking oil spray.
Stir the breadcrumbs over medium heat, until golden, then sprinkle over the casserole.
Place the casserole into the preheated oven and bake for about 30 minutes. Enjoy!