In a medium saucepan, heat broth to boiling, stir in lentils. Reduce heat and simmer, covered, until lentils are tender, but not mushy and liquid is absorbed, about 45 minutes.
Saute onion, garlic, and thyme in lightly greased skillet until onions are tender, 5 to 8 minutes. Process onion mixture, lentils, and walnuts in food processor until smooth. Stir in egg and season to taste with salt and pepper.