Start by placing the cooked lentils, onion, breadcrumbs, herbs, tomato paste, egg, lemon juice, soy sauce, and seasonings into a food processor. Pulse until the mixture is combined but still slightly textured. Avoid over-processing, or the mixture may become too soft.
Preheat your oven and line a baking sheet with parchment paper. Scoop the mixture and form it into small, evenly sized balls. Place them on the baking sheet with space between each one for even cooking.
Bake the lentil meatballs until they are firm, lightly golden, and heated through. Baking gives them a crispy exterior while keeping the inside tender and flavorful.
While the meatballs bake, mix yogurt, shredded cucumber, garlic, dill, lemon juice, salt, and pepper in a bowl. Stir until smooth and creamy. Chill the sauce until ready to serve for best flavor.