Bring a large pot of lightly salted water to a boil. Add broccoli rabe and cook 1 minute. Remove to a strainer with a slotted spoon. Add potatoes to boiling water and cook 12 minutes.
Meanwhile, cut sausages on the bias into 1/4-inch-thick slices and heat oil in a large skillet over medium-high heat. Add sausage, pepper and garlic and sauté 4 minutes. Squeeze liquid from broccoli rabe. Cut into 2-inch pieces and add to skillet. Sauté 2 minutes and season mixture with 1/8 tsp of the salt.
Drain potatoes; smash with milk and remaining 1/4 plus 1/8 tsp of the salt. Divide potatoes among 4 shallow bowls. Top with sausage mixture and spoon 2 tbsp ricotta cheese on each serving.