Puy Lentils with Prosciutto

1 cup puy lentils
2 ounces prosciutto, in small dice
1 tablespoon olive oil
1 medium red onion, finely chopped
1 clove garlic, peeled and finely chopped
1 rib celery, finely chopped
8 stems flat parsley, chopped
4 medium ripe tomatoes, cored, peeled, seeded and chopped, or 1/3 cup tomato puree

Put lentils in medium saucepan with 6 cups cold water over medium high heat. Simmer, uncovered, about 45 minutes, until lentils are tender.

Combine prosciutto and olive oil in heavy casserole and place over medium high heat. Cook about 5 muniutes, until the prosciutto begins to brown. Add onion,garlic, celery, and parsley and cook about 10 minutes more, until the vegetables are tender. Add tomatoes and cook 10 minutes longer. Stir in drained lentils, season with salt and pepper to taste, and simmer gently for 10 minutes before serving.