Set the oven temperature to 375 degrees and preheat. In a small skillet, melt butter and sauté onions over medium heat until tender and lightly browned for about 5 minutes. Set aside.
Bend asparagus spears one by one to snap the bottom stalky parts off. Place into a shallow bowl and top with a little cold water. Microwave for 1-3 minutes until crisp and tender. Drain and top with ice. Set aside.
In a large bowl, place cooked onions, diced sausage, half the cheese, eggs, cream, milk, salt, and pepper. Stir. Pour into the tart shell. Sprinkle it with remaining cheese and top with asparagus spears. Bake covered with foil for 15 minutes. Remove foil and bake for another 5-10 minutes. Remove from the oven and cool for 10 minutes before serving.