Place lentils in a small pot and cover with 2 inches of water. Cover and bring to a boil, then reduce to a simmer. Cook until lentils are tender, about 30 minutes.

Meanwhile, heat oil in a large skillet over medium-high heat. Add onion and bell pepper. Cook, stirring frequently until onion is golden brown, about 8 minutes.

Add chili powder, paprika, cumin, cayenne and tomato paste and cook, stirring constantly, until spices and tomato paste are fragrant, about 2 minutes.

Add vinegar and then use a wooden spoon to scrape up any bits from the bottom of the pan. Add 2 cups water, garlic, apricots and crushed tomatoes.

Reduce heat to medium-low and let sauce simmer until it thickens, at least 30 minutes.

When lentils are cooked, drain off any excess cooking liquid and add to the pan with sauce; stir well to combine.

Mash some or all of the lentils using a wooden spoon or potato masher. Taste and adjust seasoning if needed with more salt and pepper.

Toast hamburger buns in the oven, if desired. Ladle 1 cup of the lentil mixture on each toasted bun and serve.