Preheat oven to 375 degrees.: Warmth in the kitchen signals the start, and you should smell a faint dry heat as the oven stabilizes. Preheating ensures the casserole begins cooking immediately and the top can brown properly without overcooking the center. A common mistake is putting the dish in a cold oven, which leads to longer cook time and uneven browning. If your oven runs hot or cool, use an oven thermometer to confirm the temperature.
In a small bowl, combine the tomatoes, red onion, garlic, and vinegar. Marinate in the fridge until ready to use.: The tomato mixture should smell bright, with the sharpness of red onion softened and the garlic fragrant but not overpowering. Chilling concentrates the flavors and allows the balsamic vinegar to mellow the onion and draw juices from the tomatoes. Avoid letting the mixture sit too long uncovered, or the tomatoes can become too soft. If the mix releases a lot of liquid, spoon carefully and leave most of the free liquid behind when assembling.
In a 9 x 13 baking dish toss the pesto and chicken together and place in a single layer.: You should feel the slick, herbaceous coating of pesto on each piece of cubed chicken , which smells of basil and olive oil. Spreading the pieces in a single layer helps them cook evenly and prevents steaming. If pieces are piled, the center can stay underdone while edges overcook. Uniformly sized cubes and an even layer minimize this risk.
Scoop the tomato mixture on top, disregarding any excess balsamic vinegar liquid leftover. Try to not put any of the liquid on the chicken.: Visually, the vibrant red tomatoes will form a jewel like layer that contrasts with green flecks of pesto . Leaving behind the excess liquid is critical, because too much moisture will lead to a watery casserole instead of a cohesive, baked texture. A common issue is scooping the marinade with the tomatoes, which can dilute the casserole. Use a slotted spoon if needed.
Sprinkle the cheese on top.: As you scatter the shredded mozzarella cheese , you will notice the milky aroma and the way the cheese forms a soft blanket over the tomatoes. Even distribution ensures each scoop has melty cheese. Overcrowding one area leaves others cheese sparse, resulting in uneven melt and texture.
Mix the panko crumbs and olive oil together and sprinkle on top of the cheese.: After tossing the panko crumbs with olive oil , you should see a glossy, slightly clumped crumb texture that will toast into a golden crust. The oil promotes uniform browning and prevents the crumbs from drying out. If the crumbs are dry, they can scorch quickly; if too oily, they will stay soggy. Aim for a light, even coating.
Bake for 25-30 minutes or until chicken is done. Cover with a foil tent if crumbs begin browning too much.: During baking you will hear a gentle bubbling and smell the herbal pesto mingled with toasty crumbs. The top should turn golden and the cheese should be melted and slightly bubbling around the edges. Use an instant read thermometer to ensure chicken reaches a safe internal temperature, avoiding undercooking. If the panko darkens too quickly, tent with foil to allow the center to finish without burning the crust.