Preheat the oven to 350°F and grease a mini muffin pan (or line with paper liners)
Heat the oil in a large skillet over medium high heat. Add the onions and bell peppers and cook until soft -- about 3 minutes. Remove them from the pan and set aside. Add the sausage to the skillet and cook it, breaking and crumbling with a spatula as you go. Remove it from the heat when the sausage is cooked through
In a large bowl whisk together the flour, baking powder, sugar, salt, and black pepper. Add the eggs, Greek yogurt, and buttermilk and stir until just combined. Fold in the cheese and cooked sausage, onions, and peppers.
Spoon the batter into the prepared mini muffin pan. You can fill each cup all the way to the top.
Bake for 22 minutes. let them cool for at least 5 minutes. Use a butter knife to run around the edges of each omelet muffin to loosen it from the pan.
Serve warm or let them cool completely and then wrap individually to refrigerate or freeze for later reheating.