Preheat oven to 400 degrees F. Line a baking sheet and a 9x13 baking pan with foil.
Place chicken breasts or thighs in the lined baking pan. Mix together olive oil, lemon juice, salt and pepper, and brush over chicken. Bake for ~30 minutes or until the internal temperature is 165 degrees F.
Toss bell peppers and tomatoes in olive oil, salt and pepper. Transfer to the lined baking sheet and bake for 20-25 minutes or until the vegetables are slightly blistered.
While the chicken and veggies are roasting, prepare the quinoa. Bring the dry quinoa and water to a boil, cover, and bring to simmer. Cook for ~15 minutes or until all of the water is absorbed.
Next, prepare the tzatziki. Mix the yogurt, lemon juice, garlic, cucumber, dill, and salt and pepper in a medium bowl until well combined.
Assemble the bowls. Depending on how much chicken you make, you should have enough for 4-6 bowls. Add a piece of chicken, a scoop of quinoa, a portion of vegetables, and a scoop of tzatziki to containers. Refrigerate and enjoy all week long! You may have leftover tzatziki and quinoa.
To make this vegetarian, simply omit the chicken and replace with beans or lentils.