Sausage and Potato Roast with Arugula

lbs Sweet italian sausage

(about 1/2 pound) unpeeled shallots, medium

lbs Potatoes, mixed red

Kosher salt and freshly ground black pepper or red pepper flakes

Lemon juice or white wine vinegar, fresh

Olive oil

One 5-ounce package of baby arugula or one 8-ounce bundle, stemmed and chopped