Fill a pot or saucepan halfway with water. Bring the water to a low simmer. Add vinegar to the water.
Crack the eggs into a bowl. Gently place the eggs into the water. (Poach multiple eggs at once, being careful not to overcrowd the pan.)
In the meantime, heat the oil and butter in a non-stick pan over medium heat. Once hot, add the mushrooms, scallions, and garlic. Season well with salt and a little pepper. Sauté until golden brown and tender, about 4-6 minutes. Then spread on the toasted bread.
Cook the eggs in the water for 3-5 minutes, gently flipping over halfway through until the whites are firm. Remove the eggs with a slotted spoon onto paper towels to drain any excess water. Season with salt and pepper.
While the eggs are cooking, brown the chicken sausage on both sides in a non-stick pan over medium heat.
Serve the eggs over the mushrooms with chicken sausage, and plums. Garnish with scallions.