Preheat the oven to 200 degrees. (if making multiple omelets keep cooked omelets in the oven on a baking sheet until all are complete)
In a nonstick pan, heat olive oil over medium heat. Saute the chicken sausage until nicely browned, add in the spinach till wilted, then add in tomatoes and egg.
Toast the bagel and spread with cream cheese.
After egg mixture begins to set around edges, about 30 seconds, using a heat-safe spatula, carefully lift the egg from the side and tilt the skillet, so uncooked egg can reach the bottom of the pan. Repeat this process, tilting as necessary until the egg is no longer runny.
Add feta cheese and fold the omelet in half. Serve with bagel and watermelon.