Cook the quinoa. Rinse quinoa under cold water. Bring broth to a simmer in a medium pot, add quinoa and salt, cover, and cook on low for 15 minutes.
Turn off heat and let it sit, covered, for 5 minutes. Fluff with a fork and add lemon juice if using.
Make the yogurt sauce. In a small bowl, mix yogurt, lemon juice, zest, garlic, dill, salt, and pepper. Adjust lemon or salt to taste.
Chill until serving.
Mix the meatballs. In a large bowl, combine turkey, onion, garlic, panko, egg, parsley, dill, oregano, lemon zest, cumin, salt, and pepper. If it feels too firm, add 1–2 tbsp water to keep them tender.
Shape the meatballs. Form 18–20 small meatballs (about 1.5 tablespoons each). Aim for even size so they cook uniformly.
Cook the meatballs. Heat a large nonstick skillet over medium with a light spray of olive oil.
Add meatballs in a single layer without crowding. Cook 8–10 minutes, turning every few minutes, until browned and the center reaches 165°F (74°C). Cook in batches if needed.
Alternatively, bake at 400°F (205°C) for 12–15 minutes on a lined sheet, flipping once.
Toss the cucumber-tomato mix. In a bowl, combine tomatoes, cucumber, red onion, olive oil, lemon juice, salt, and pepper. Taste and adjust.
Assemble for meal prep. Divide quinoa into 4 containers. Top with meatballs (4–5 each).
Add the cucumber-tomato mix on the side and a dollop of yogurt sauce in a separate small cup or add just before eating. Add greens, olives, or feta if you like.