Step 3: Layer loosely in the dish
Transfer the mixture into the casserole dish without packing it down. Scatter the butter pieces throughout, making sure some are tucked between layers.
Step 4: Slow bake covered
Cover tightly with foil and bake for 1 1/2 hours. During this stage, the vegetables soften and begin to meld together in their own juices and melted butter.
Step 5: Caramelize uncovered
Remove the foil and raise the oven temperature to 375°F. Bake uncovered for another 45–60 minutes.
Look for: Deep golden-brown color on top and bubbling, caramelized edges where the sugars have concentrated.
Step 6: Rest before serving
Let the dish sit for 15–20 minutes before serving.
Tip: This resting time helps the juices settle and thicken so the casserole holds together when scooped.
