preheat the oven to 400°

heat the oil, if using, over medium-high heat, add sausages, and cook until browned

remove the sausages and set them aside on a plate lined with a paper towel

reduce the heat to medium

leave the oil in the pan from the sausages, or the vegetable oil if you used it

add the potatoes and cook for 1-2 minutes

then add the red peppers and onions

cook and stir for another 2 minutes

sprinkle with garlic, salt, and pepper over the potatoes

cook another 2 minutes

and add the spinach on top, stir it in, and let it cook until soft and wilted

taste and add more salt or pepper if needed

once it's wilted sprinkle the cheese on top of the potato mix

beat the eggs and cream or milk together and pour the egg mixture over the potato mixture

turn the heat to low and let it cook for 4 minutes, do not stir

then slide the skillet into the preheated oven and bake for 10-12 minutes or until the frittata is set and cooked through

check for doneness at the 10-minute mark by cutting the center of the frittata to check for runny eggs

if you see raw eggs bake again for 2 more minutes until the eggs are cooked through