Preheat oven to 350°F. Prepare a 9-inch pie pan with a spritz of nonstick cooking spray.
In a medium skillet over medium heat, cook the sausage while breaking up clumps with a wooden spoon until it's not quite completely browned, about 7-9 minutes. When it's almost cooked through, add the spinach. Cook and stir until the spinach wilts, about 1 to 2 minutes. Transfer this mixture to the prepared pie pan and set aside.
In a medium-sized mixing bowl, add the eggs, half-and-half, dry mustard, salt, and pepper. Whisk until all the ingredients are well blended.
Stir the cheese into the egg mixture and pour it over the sausage mixture.
Bake until the quiche appears firm with only a slight jiggle in the center when you gently shake the pan, and a knife inserted in the center comes out clean or almost clean, about 45-50 minutes.
Remove from the oven and let the quiche rest for at least 10 minutes before slicing.
Garnish with sliced avocados if desired and serve warm.