Start the quinoa so it has time to rest and cool slightly: rinse 2/3 cup dry quinoa in a fine mesh sieve under cold water (rinsing removes the bitter saponins and prevents a soapy taste). Combine the rinsed quinoa and 1 1/4 cups water in a small saucepan, bring to a simmer, cover, and cook gently for 12–15 minutes until the water is absorbed. Remove from heat and let sit, covered, for 5–10 minutes to steam — this makes the grains fluffy rather than gummy. Fluff with a fork and set aside.

Marinate the chicken to infuse flavor and keep it juicy: pat the 4 chicken breasts dry (a dry surface browns better), then mix 1 tbsp olive oil, 2 tbsp lemon juice, 1 minced garlic clove, 1/2 tsp salt, 1/4 tsp black pepper and 1/2 tsp dried oregano. Toss the chicken in the marinade and let sit for at least 10 minutes at room temperature (or up to 2 hours refrigerated). The acid brightens flavor and the short marinade time prevents the meat from becoming tough.

Preheat your oven to 425°F (220°C) for the vegetables and preheat a grill pan or skillet over medium-high heat for the chicken. High oven heat and a hot pan both create quick surface caramelization — that char and Maillard flavor is what makes the vegetables and chicken taste like restaurant food.

Toss the chopped bell pepper, zucchini and red onion (≈4 cups total) with 1 tbsp olive oil, 1/2 tsp salt, 1/4 tsp black pepper and 1/2 tsp dried oregano on a rimmed baking sheet. Spread into a single layer (crowding steams instead of roasts). Roast on the top rack for 15–18 minutes, tossing once halfway, until edges are browned and vegetables are tender. Roasting concentrates sweetness and gives a smoky note that contrasts the bright lemon in the dish.

While the veg roast, make the tahini‑lemon dressing: in a bowl whisk together 4 tbsp tahini, 2 tbsp lemon juice, 2 minced garlic cloves, 1/4 tsp salt, 1/4 tsp black pepper and 2 tbsp warm water to start. Whisk until smooth; thin gradually with an additional tablespoon or two of warm water to reach a pourable consistency. Tasting and adjusting at this stage ensures the dressing is balanced — tahini needs acid and a little salt to sing.

Cook the chicken: remove breasts from marinade and pat lightly to remove excess moisture (this helps a good sear). Heat your grill pan or skillet until hot; add a light film of oil or spray. Sear chicken 3–4 minutes per side until deeply golden, then reduce heat to medium and cook until an instant-read thermometer reads 160°F (71°C) — carryover will bring it to 165°F. Rest the chicken 5 minutes before slicing; resting redistributes juices so slices stay moist.

Finish and warm the quinoa: if the quinoa has cooled too much, warm gently in a skillet with a teaspoon of olive oil and a pinch of salt to revive its texture. Stir in half of the chopped parsley to add a fresh herb note into the grain base.

Assemble the bowls: divide the warm quinoa among 4 bowls (about 1/2 cup cooked per serving). Top each with 1 cup roasted vegetables, a handful (1 cup) of baby spinach, and 1/2 cup halved cherry tomatoes. Slice the rested chicken thinly and fan over each bowl (1 chicken breast / 4 oz per bowl). Drizzle 1 tbsp tahini dressing over each bowl (divide the dressing evenly). Sprinkle remaining parsley and a pinch of crushed red pepper if using.

Final seasoning and serve: finish with a quick squeeze of lemon if you like it brighter and a final crack of black pepper. Serve immediately so the chicken remains warm and the spinach has a little wilt from the hot quinoa and veg for contrast in texture.