In a large saucepan, add the lentils and enough water to cover by 2 inches. Bring to a boil, then reduce the heat to medium and simmer, stirring often, until tender, 20 to 25 minutes. Stir in the vinegar and olive oil; season with salt and pepper.
In a large bowl, combine the salmon, bread crumbs, egg and onion; season with salt and pepper. Stir with a fork until the mixture just comes together. Using wet hands, form into four 3/4-inch-thick cakes.
Place the flour on a plate and lightly coat the salmon cakes on both sides. In a large nonstick skillet, melt the butter over medium heat. Add the salmon cakes and cook, turning once, until lightly browned, about 10 minutes. Set aside and cover loosely with foil.
Wipe out the skillet with a paper towel and add the bell pepper, skin side down. Cover and cook over medium heat, turning occasionally, until the skin is charred and blistered and the flesh is tender, 6 to 8 minutes. Remove the pepper from the heat and let cool about 5 minutes. Using a paper towel, remove the blackened skin and cut the flesh into thin strips. Divide the lentils among 4 serving plates; top each with a salmon cake and the pepper strips.