This is a colorful, delicious pasta dinner with a healthy dose of Swiss chard.
Bring a large pot of salted water to a boil. Meanwhile, in a food processor, pulse garlic until finely chopped. Add bread and Parmesan; season with salt and pepper. Pulse until coarse crumbs form. In a large skillet, melt 1 tablespoon butter over medium heat. Add crumbs and cook, tossing, until browned, 6 to 8 minutes. Transfer crumbs to a bowl, and set aside; wipe skillet clean.
Cook pasta in boiling water until al dente, according to package instructions. Reserving 1 cup pasta water, drain pasta, and return to pot. Set aside.
While pasta is cooking, melt 1 tablespoon butter in skillet over medium heat. Add onion; cook until lightly browned, 6 to 8 minutes. Add chard stalks and 1/2 cup pasta water; cook, stirring often, until beginning to soften, about 4 minutes. Add leaves, and cook until tender, 4 to 6 minutes more. Stir in vinegar.
Add chard mixture and remaining tablespoon butter to pasta. Season with salt and pepper; toss to combine. Thin with pasta water, if desired. Serve topped with breadcrumbs and additional Parmesan.
It's best to use green Swiss chard in this recipe; the red variety will turn the pasta pink.