Preheat oven to 350 degrees F. Coat a 2-quart rectangular baking dish with cooking spray. In a medium bowl, whisk together the eggs, milk, salt, and pepper. Pour into prepared dish. Bake for 20 to 25 minutes or until set.
Meanwhile, cut smoked sausage into 6 pieces. Halve each piece lengthwise, cutting to but not through the other side to keep the pieces together. Heat a 12-inch skillet over medium heat. Add sausage pieces to skillet, opening each like a book. Cook for 4 to 6 minutes or until browned, turning once. Remove and keep warm.
In the same skillet, heat butter until melted. Add onion. Cook and stir over medium heat for 6 to 8 minutes or until just browned and tender.
To assemble, spread cut sides of muffins or biscuits with horseradish. Cut baked egg mixture into 6 squares. On muffin bottoms, layer cheese, spinach, egg, sausage, and onion. Add muffin or biscuit tops.
Instead of toasted English muffins, you can substitute baked large biscuits, split.
Nutrition Facts (per sandwich): 571 cal., 37 g total fat (13 g sat. fat), 295 mg chol., 1418 mg sodium, 37 g carb., 2 g fiber, 5 g sugar, 28 g pro.
Daily Values: 22% vit. A, 14% vit. C, 37% calcium, 17% iron