Spray a small sauté pan with cooking spray, and heat over medium high heat. Dice sausage into small disks and cook breakfast sausage along with diced onion until they are browned.
Meanwhile, mix the rest of ingredients, (besides the toppings) in a medium bowl, whisking until they are combined. Spray crockpot with cooking spray and pour mixture into crockpot. Sprinkle the browned sausage and onion over the egg mixture. Cover crockpot and cook 2.5-3 hours on low or on high for 1.5- 2 hours. Omelet is done when eggs are set.
If using cheese, sprinkle with the omelet with shredded cheese in the crockpot and replace cover; let stand 2 to 3 minutes or until cheese is melted. Turn off the slow cooker. Cut the omelet into 6 wedges and garnish with diced tomato if desired. Enjoy!