Heat olive oil in large skillet over medium heat.
Add sliced onions and cook 20 minutes until golden.
Stir occasionally to prevent burning onions completely.
Steam broccoli florets until bright green and tender.
This takes about 5 minutes in steamer basket.
Drain well and set aside for later use.
Preheat oven to 375°F for perfect baking temperature.
Grease baking dish with butter to prevent any sticking.
Layer steamed broccoli in prepared baking dish evenly.
Top with caramelized onions spreading them out completely.
Pour heavy cream over vegetables in dish slowly.
Sprinkle Gruyere cheese over entire casserole surface generously.
Add breadcrumbs on top for extra golden crunch.
Season with salt and pepper to taste preferences.
Bake 25 minutes until bubbly and golden brown.
Let cool 5 minutes before serving this delicious.