Preheat oven to 400 degrees F (205 degrees C).
Into a mixing bowl, add Heavy Cream (3/4 cup), Egg (1), and Milk (1/2 cup). Whisk to combine.
Add Dried Rosemary (1 teaspoon), Shallot (1), Salt (1 teaspoon), Ground Black Pepper (1/2 teaspoon), and Garlic (1 tablespoon). Mix well.
Add Onion (1) and Ricotta Cheese (1 cup). Mix.
Add Frozen Spinach (1 cup). Mix.
Add Potato (5). Mix well to coat.
Into an 8 by 11-inch casserole dish, add the Butter (4 tablespoon). Swirl it around so the butter covers the bottom. Transfer the mixture to the dish. Sprinkle the top with Panko Breadcrumbs (to taste), then Cheddar Cheese (1/2 cup).
Cover dish with foil. Bake for 30 minutes.
Remove the foil. Bake for another 30 minutes.
Allow dish to rest for 15 minutes before serving.