Heat oil in a large saucepan over medium-high heat. Add onion and sauté until softened, 1 min. Stir in tomatoes, wine, sorted mussels, salt and pepper. Cover and cook until mussels open, approx. 7 min.
Divide mussels among 4 bowls, discarding any unopened shells. Stir pesto and lemon juice into reserved broth, ladle over mussels and serve with bread.
They may look fancy, but pulling together a mussels main is fast, easy and inexpensive.