Add more oil to the skillet if it looks dry, then add onions. Cook, stirring occasionally, until onions are tender and golden, 10 to 15 minutes. Don't rush this step, you want the onions to be soft and sweet. Add 1 cup dry white wine and cook down until reduced by 1/2. Stir in the mustard. Return sausage to pan and stir to heat through. Spoon polenta into bowls and top with sausage and onion, garnished with rosemary and cherry tomatoes.