For the chicken sausage: If raw, cook in a 400 degree oven to an internal temperature of 160 degrees using a thermometer. If already cooked: Preheat a non-stick pan over medium heat and brown the chicken sausage slices on each side.
In a saucepan over medium heat, melt the butter. Add the flour and cook until it starts to turn golden brown. Stir in the milk and bring the mixture to a simmer. Stir occasionally, until the mixture thickens slightly, about 3-5 minutes. Add the spinach and cook until the spinach has wilted (about 1-2 minutes). Remove the pan from the heat and stir in the parmesan cheese. Season with salt and pepper, to taste. Cover with a lid on to keep warm. (If the sauce is too thick, thin it out with a little more milk).
Fill a saucepan halfway with water. Bring the water to a simmer. Stir the vinegar into the water. Crack the eggs into a bowl. Gently place the eggs into the water. (You can poach multiple eggs at once, being careful not to overcrowd the pan.) Cook the eggs in the water for 3-5 minutes, gently flipping over about halfway through until the whites are firm. Remove the eggs with a slotted spoon onto a paper towel to drain excess water. Season with salt and pepper.
Toast the english muffins.
Spoon the spinach sauce over the english muffins, top with eggs. Serve with chicken sausage and oranges.