Preheat the oven to 400 degrees F. Grease a 9-by-13-inch casserole pan with cooking spray.
Set a medium pan over medium heat and add 1 tablespoon olive oil. Add the sausage and cook, breaking it up with the back of a wooden spoon or spatula, until cooked through and beginning to brown, about 5 minutes. Remove to a bowl and set aside. Add the onion to the pan and saute until translucent and the browned bits of sausage in the pan have been picked up by the onion, about 5 minutes -- this will add lots of flavor! Once the onions are cooked, add the potatoes. Cook until they are tender and just beginning to brown, about 7 minutes. If necessary, add more olive oil so the potatoes don't burn or stick to the bottom of the pan. Remove the potatoes and onions to the bowl of sausage. Coat the pan with a bit of olive oil and add the broccoli. Saute until just tender, about 5 minutes. Season with salt and pepper. Transfer to the sausage bowl.
Add the milk, cream, eggs and some salt and pepper to a separate medium bowl and whisk to combine.
Transfer the sausage and vegetable mixture to the prepared casserole pan. Toss and distribute evenly on the bottom of the pan. Gently pour the egg mixture over the top. Bake until the frittata is puffed and beginning to brown and the egg is set in the middle, about 30 minutes. Allow to set for 5 minutes before slicing. Garnish with the dill and serve. The frittata is delicious hot out of the oven, room temperature or even cold!