Prepare the sweet potatoes:
Preheat your oven to 375°F (190°C).
Peel and cut the sweet potatoes into cubes.
Boil the sweet potato cubes in salted water until tender, about 10 to 15 minutes. Drain and set aside.
Sauté the spinach:
Heat a little olive oil in a large skillet over medium heat.
Add chopped onion and sauté until translucent.
Add minced garlic and cook for another minute.
Add chopped spinach and cook until wilted. Season with salt, pepper and dried thyme (if using). Remove from heat.
Make egg mixture:
In a large bowl, beat eggs.
Add milk (if using) and mix well.
Season with a pinch of salt and pepper.
Assemble casserole:
In a large baking dish, evenly distribute boiled sweet potato cubes.
Add sautéed spinach and onion mixture over sweet potatoes.
Pour egg mixture over vegetables.
Sprinkle grated cheese evenly over top.
Bake:
Bake in the preheated oven for 25 to 30 minutes, or until the cheese is melted and golden and the eggs are set.
