Prep everything first — rinse and pat the chicken dry, slice the zucchini, bell pepper and cube the eggplant. Reason: mise en place keeps the cook time tight and prevents overcooking the chicken while you chop vegetables. Patting chicken dry is essential to get a good sear and predictable grill time.

Season the chicken with 3/4 tsp kosher salt and 1/4 tsp black pepper. If you have time, dry-brine the breasts for 20–30 minutes in the fridge uncovered — this concentrates flavor and helps the meat retain moisture when grilled. Reason: salt draws juices to the surface then reabsorbs them seasoned, improving juiciness.

Preheat the oven to 220°C (425°F). Place a rimmed baking sheet or roasting pan in the oven to heat while you finish prepping the vegetables. Reason: a hot sheet gives the vegetables an immediate sear when they hit the pan, promoting caramelization rather than steaming.

Toss the cubed eggplant, zucchini slices and bell pepper strips with 2 tbsp olive oil, 1/4 tsp kosher salt and 1/4 tsp black pepper in a large bowl so each piece is lightly coated. Spread them in a single layer on the preheated baking sheet (use two pans if crowded). Reason: oil promotes browning and flavor; an even, single layer ensures hot air reaches each piece so they roast rather than steam.

Roast the vegetables for 20–25 minutes, turning once halfway through. Look for deep golden edges on the eggplant and slight charring on pepper skin. Reason: the longer roast time concentrates sweetness and reduces eggplant bitterness; turning exposes all sides to the hot pan for even browning.

While the vegetables roast, make the almond pesto. Toast the almonds in a dry skillet over medium heat for 3–4 minutes, shaking often, until they smell nutty and are just golden. Cool slightly. Reason: toasting amplifies almond flavor and gives the pesto depth — don't let them burn or the pesto will taste bitter.

In a food processor, pulse the toasted almonds, 1 packed cup basil, 1 garlic clove, 2 tbsp olive oil, 1/4 tsp kosher salt and 1/8 tsp black pepper until a rustic paste forms. If the pesto seems too thick, add 1–2 tsp water or a squeeze (about 1/2 tsp) of lemon juice to loosen it. Reason: processing releases oils and creates a creamy binder; a slightly coarse texture gives pleasant contrast to the roasted vegetables.

Preheat a grill pan (or outdoor grill) over medium-high heat. Brush the chicken lightly with 1 tbsp olive oil to help conduct heat and prevent sticking. If the breasts are uneven in thickness, pound them to an even 1/2–3/4" thickness with a meat mallet or rolling pin (place in a zip-top bag first). Reason: an even thickness cooks uniformly so the outside doesn't overcook before the center reaches safe temperature.

Grill the chicken 4–6 minutes per side (depending on thickness) until a deep golden-brown and the internal temperature reaches 74°C (165°F). Let the chicken rest 5 minutes before slicing. Reason: high heat gives a golden crust and grilled flavor; resting lets the juices redistribute so slices stay moist.

Taste the roasted vegetables and finish with a pinch of salt and a drizzle (about 1/2 tsp) of olive oil if needed. Spoon 2–3 tbsp of almond pesto over the warm vegetables and toss lightly to coat — reserve a little pesto for the chicken. Reason: warming the pesto briefly releases aromatics and helps it melt into the vegetables without wilting the basil.

Slice the rested chicken on the bias into 1/4" slices. Plate the roasted vegetables in a shallow mound and fan the chicken slices over them. Finish with a few small dollops of pesto, a crack of black pepper and an optional squeeze of lemon. Reason: slicing on the bias looks elegant and gives longer pieces that are easy to pick up with vegetables; citrus brightens the rich pesto.

Serve immediately. Leftover pesto keeps up to 3 days in the refrigerator in a sealed container covered with a thin film of olive oil, or freeze in ice cube trays for single-use portions. Reason: oil prevents oxidation and preserves the pesto's bright color and flavor.