Baked Spinach Ravioli

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4 tablespoons unsalted butter

1 small onion, finely chopped

2 cloves garlic, minced

2 sprigs fresh rosemary

1/4 cup all-purpose flour

2 cups whole milk

Kosher salt and freshly ground pepper

2 9-ounce packages refrigerated cheese ravioli

1 10-ounce package frozen chopped spinach, thawed and squeezed dry

2 ounces low-moisture mozzarella, cut into 1/2-inch cubes

1/4 cup grated grana padano or parmesan cheese