Cook the brown lentils according to package instructions until tender but not mushy. Drain any excess water and let them cool slightly. The lentils should be soft yet still hold their shape.
In a large bowl, mash the cooked lentils slightly using a fork or pulse briefly in a food processor. Add red onion, garlic, parsley, oregano, salt, pepper, olive oil, breadcrumbs, and egg. Mix everything until well combined but not overworked.
Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. Scoop about 1–2 tablespoons of mixture and roll into small balls. Place them evenly spaced on the baking sheet.
Bake for 25–30 minutes, flipping halfway through for even browning. The outside should be golden and slightly crisp. Let them cool for a few minutes before serving.
While the meatballs bake, grate the cucumber and squeeze out all excess liquid using a towel. In a bowl, mix Greek yogurt, cucumber, olive oil, lemon juice, garlic, dill, and salt. Stir until smooth and refrigerate until ready to serve.