Preheat oven to 350°. In a large nonstick skillet, heat oil over medium-high heat. Add onion and sweet pepper; cook and stir until tender, 3-4 minutes. Add spinach; cook 1 minute longer. Remove from heat; let cool 5 minutes. In a large bowl, whisk eggs, salt, pepper and onion mixture. Divide egg mixture among 6 greased 4-in. muffin top tins. Bake until eggs are set, 12-15 minutes., Meanwhile, shape sausage into six 5-in. patties. In the same skillet, cook patties over medium heat until a thermometer reads 160°, 4-5 minutes on each side. Drain on paper towels if necessary. Layer sandwich bottoms with sausage patties, egg rounds and cheese; replace tops.