Preheat the oven to 190 degrees. Wash the red pepper, slice it in half lengthwise and remove the seed membranes and seeds. Peel the clove of garlic and chop or press it finely. Mix the cooked lentils with the chopped walnuts, the finely chopped garlic, the teaspoon of cumin and the tablespoon of olive oil thoroughly together in a bowl until a cohesive filling is formed.
Distribute the lentil mixture evenly over the two hollowed pepper halves and press the filling down lightly. Place the stuffed peppers side by side in a suitable baking dish. Slide the dish into the middle of the preheated oven and bake for approximately 25 to 30 minutes, until the pepper is completely tender.
Remove the baking dish from the oven once the peppers are cooked and the walnuts on top take on a slightly darker color. Let everything rest for a few minutes on the counter so the worst of the heat dissipates. Transfer the warm pepper halves to the plates and serve immediately.