Heat oil in a pan over medium-high heat. Saute onion in the hot oil until softened, about 5 minutes. Add spinach and beaten eggs and mix together to coat. Add cheese and cover the pan with a lid.
Lower heat and steam until eggs are set, about 5 minutes.
Meanwhile, microwave leftover mashed potatoes until heated through, about 1 to 2 minutes.
Lay out cooked omelette over potatoes and top with salsa. Add salt and pepper to taste.