Preheat oven to 400 degrees. Cut the cherry tomatoes in half, place in a baking dish. Drizzle with olive oil, then shimmy the dish around to get the tomatoes coated with the olive oil. Season well with salt and pepper. Bake for 15-20 minutes. Remove from oven and reduce heat to 350 degrees.
Meanwhile, heat a little olive oil in a skillet over medium heat. Add the onion and saute 1 min just to soften. Add the sausage and continue cooking about 5 minutes until beginning to brown. Remove from heat.
In a large batter bowl with a spout, whish together eggs, milk/cream, baking powder and smoked paprika. Whisk well so whites and yolks are fully incorporated but don't overdo is. Season with salt and pepper. Stir in the sliced mustard greens.
Spray your oven-safe frittata baking dish with non-stick cooking spray. Scatter the roasted cherry tomatoes in the dish, followed by the onion/sausage mixture. Pour the egg mixture over the top and shimmy the dish to ensure the egg mixture filters down over the ingredients.
Bake at 350 degrees for 15 minutes, and then check at 5 minute intervals until the mixture doesn't jiggle at all when you shimmy the dish. Remove from oven and wait 5 minutes before slicing. Serve and enjoy!