Preheat and prep: Preheat the oven to 400°F (200°C). Line a rimmed baking sheet with parchment or lightly oil a shallow baking dish. Preheating to a relatively high temperature gives the zucchini/squash edges a chance to roast and caramelize quickly without overcooking the interior.

Rice — rinse and cook: Rinse 1 cup long-grain rice under cold water until the water runs clear (this removes surface starch so the grains stay separate). In a small saucepan, bring 1.5 cups chicken broth and a pinch of salt to a boil. Add the rinsed rice, stir once, reduce to a low simmer, cover and cook 15–18 minutes until the liquid is absorbed. Let rest covered for 10 minutes and then fluff with a fork. Cooking rice in broth infuses it with savory flavor and letting it rest keeps the grains fluffy rather than sticky.

Prepare the vegetables: While the rice cooks, halve the zucchini and squash lengthwise and scoop the centers with a spoon to form 'boats', leaving about a 1/2-inch shell (reserve the scooped flesh). Lightly salt the hollowed boats and set them cut-side up on the prepared sheet (the salt draws out excess moisture and helps them roast instead of steaming). Roughly chop the reserved zucchini/squash flesh and set aside.

Sear the chicken: Pat the chicken breasts dry and cut into 1/2-inch dice. Heat 1 tablespoon olive oil in a large skillet over medium-high heat until shimmering. Add the diced chicken in a single layer (don’t overcrowd — work in batches if needed) and season with salt and pepper. Sear without moving for 2 minutes to develop color, then stir and cook until cooked through (about 4–5 minutes total). Remove the chicken to a bowl and keep warm. Searing builds a fond (browned bits) on the pan that deepens flavor for the filling.

Sauté aromatics and vegetables: In the same skillet, reduce heat to medium and add 2 tablespoons olive oil and 1 tablespoon butter. Add the diced onion and a pinch of salt; cook gently 5–7 minutes until softened and starting to caramelize — this sweetness balances the savory notes. Add the minced garlic and chopped reserved zucchini/squash flesh and cook 2–3 minutes until softened. Add the chopped broccoli florets and cook another 3–4 minutes until bright green and just tender. Browning the onion and sweating the vegetables concentrates sweetness and removes excess moisture so the filling isn’t watery.

Combine filling: Lower heat to medium-low. Return the cooked chicken to the pan, add the fluffed rice, grated Parmesan, chopped parsley, thyme, lemon zest (if using), and the beaten egg. Stir thoroughly to combine and heat through — the egg helps bind the filling so it holds in the boats after baking. Taste and adjust seasoning with salt and pepper. The egg also gives the filling a silkier texture when baked.

Fill the boats: Spoon the chicken-and-rice mixture into each zucchini and squash cavity, pressing gently to mound the filling. Brush the exposed edges of the squash with a little olive oil and (optional) brush the tops of the filling lightly with the remaining beaten egg for a glossy, golden finish. The oil on the edges promotes even browning; the egg wash encourages a golden crust over the top.

Bake: Place the filled boats on the prepared sheet and bake at 400°F (200°C) for 18–22 minutes until the vegetables are tender-crisp and the filling is heated through and lightly browned on top. If you want a deeper crust, switch to broil for 1–2 minutes at the end — watch closely to prevent burning. Baking at this temperature gives a roasted edge while preventing the shells from collapsing.

Finish and rest: Remove from the oven, let rest 5 minutes to allow juices to redistribute (this helps the filling set). Garnish with extra chopped parsley and an extra sprinkle of Parmesan if desired. Resting makes the boats easier to serve and avoids the filling spilling out immediately.

Serve: Plate one zucchini and one squash boat per person or split as preferred. Serve with a lemon wedge on the side for a bright squeeze, or drizzle a teaspoon of extra-virgin olive oil over the top. The acid cuts the richness and lifts the flavors.