Spinach Artichoke Strata

INGREDIENTS

2 tbsp

olive oil, plus more as needed

yellow onion, sliced

Kosher salt and freshly ground pepper,

1 lb

fresh spinach

1 15 ounce can

artichoke hearts, drained and quartered

1 1/2

-lb. loaf french bread, bread torn or cut into 1-inch squares and toasted (about 9 cups)

2 cups

Gruyère cheese, grated

1 cup

Parmesan cheese, grated

large eggs

2 1/2 cups

heavy cream