Slice chicken in half lengthwise to make 4 thin pieces. Pound to even thickness. In a bowl, whisk together mustard, balsamic, lemon juice, garlic, paprika, Italian seasoning, olive oil, ½ tsp salt, and ¼ tsp pepper. Set aside ⅓ cup of marinade. Pour the rest over chicken in a resealable bag, seal, and refrigerate 30 mins to 4 hours.
Clean and oil grill grates. Preheat to medium heat (375–450°F). Grill chicken 4–6 mins per side, or until internal temp reaches 160°F. Brush with 2 tablespoons of reserved marinade while grilling. Save remaining marinade for serving. (For stovetop instructions, see note 2.)
Toppings
In a bowl, combine tomatoes, garlic, onion, basil, vinegar, and oil. Season to taste. Let sit 10–15 mins at room temp to marinate, tossing occasionally.
Cook quinoa per directions. Let rest, fluff with a fork, and slightly cool. Drain extra liquid from the bruschetta over the quinoa and toss to coat.
Assemble
Divide quinoa among 4 plates. Add chicken, brush with remaining marinade, and top with bruschetta. Add avocado and finish with grated Parmesan and extra basil, if desired.
Video
Notes
Note 1: Finely grate a block of Parmesan right over plates with a microplane for the best texture.Note 2: For stovetop: Heat 1 tbsp oil in a large nonstick skillet over high heat. Let excess marinade drip off, then cook chicken 2–4 mins per side until done. Remove from skillet and brush with reserved marinade.Storage: Store leftovers in airtight containers in the fridge for 3-5 days. Keep the chicken, bruschetta, and quinoa separate. Add avocado just before serving.