Preheat oven to 400° + lightly oil baking dish; heat 1 tbsp oil in skillet over med-high; add half the bag of broccoli, salt/pep + sauté 4-5 min until lightly browned; add 2 tbsp water, reduce heat, cover + steam 3-5 min more until tender; set aside

Thinly slice onion; wipe mushrooms clean + thinly slice; heat 2 tbsp oil in skillet over high, add mushrooms, onions + cook undisturbed 3 min; sauté 3-4 min until browned + tender, season with salt/pep

Add veggies to baking dish, drizzle with 6 tbsp sauce + sprinkle on ½ cup cheese; bake 12-15 min until hot + cheese is melty