For the sausage: cook in a 400 degree oven on a baking sheet to an internal temperature of 160 degrees using a thermometer.
In a bowl mix the cauliflower, paprika, cheese, scallions, and garlic powder. Season well with salt and a little pepper. Mix in the flour to bind the mixture together.
Heat a non-stick pan over medium heat. Once hot, add the oil. Add the cauliflower mixture to the pan. Use a spatula to neatly flatten it out on the surface of the pan. Cook until the bottom is golden brown and crispy. Remove from the heat, place a large plate or cutting board over the top of the pan and invert the pan so the rosti is on the plate. Then slide the uncooked side of the rosti back into the pan and cook until golden brown and crispy. Then remove from the heat.
Fill a saucepan halfway with water. Bring the water to a simmer. Stir the vinegar into the water. Crack the eggs into a bowl to make sure the yolks dont break. Stir the simmering water in a circular motion then 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.
Serve the eggs over the rosti with sausage, toast, and watermelon. Garnish with scallions.