Place water, lentils, and bouillon cubes in a pot; cover and simmer over medium heat until tender, about 30 minutes. Drain well.

Heat oil in a skillet over medium heat; add onion and sauté until soft and translucent.

Blend cooked lentils, onions, and walnuts in a food processor until it reaches the desired consistency for pâté. Season to taste with salt and pepper. Chill before serving.