Bring 2.5 cups water to boil in a pot, add rinsed lentils and diced onion, then simmer for 15 minutes until lentils are tender but hold their shape.
Heat olive oil in a separate skillet, mince garlic and sauté for 30 seconds, then add spinach and cook for 2 minutes until wilted.
Drain any remaining liquid from the lentils, stir in spinach, canned tomatoes, cumin, and oregano, then simmer for 3 minutes.
Warm pita bread in the oven or microwave, then cut in half and fill each with lentil-spinach mixture, top with cucumber slices and tomato if desired.