Heat the oven to 200°C (400°F). Toss the squash with the olive oil, salt, pepper, and maple syrup, if using.
Heat the skillet over medium-high heat for 2 minutes. Add the squash in a single layer, cut-side down. Sear for 4 minutes without moving it, until the cut faces are deep golden and release easily from the pan. Flip the pieces and cook 1 minute more.
Move the skillet to the oven and roast for 12 minutes, until a knife slides into the squash with a little resistance. It will finish cooking with the fish.
While the squash roasts, pat the tilapia dry again and season with the pepper and lemon zest. Fold each fillet's thin tail end under so it is an even thickness. Wrap each fillet with one or two slices of prosciutto, overlapping them slightly.
Take the skillet out of the oven and push the squash into a loose layer. The handle will be hot, so use a mitt. Set the wrapped fillets on top, seam-side down.
Bake for 10 to 12 minutes, until the fish is opaque, flakes when pressed with a fork, and reads 63°C (145°F) at the thickest part. The prosciutto should look dry and a little crisp at the edges. If the top is pale, switch the oven to broil for 1 minute and watch closely.
While the fish bakes, melt the butter in a small pan over medium heat. Add the sage leaves and cook for 1 to 2 minutes, until the butter foams, turns light brown, and smells nutty and the leaves go crisp. Take it off the heat and stir in the lemon juice.
Spoon the sage butter over the fish and squash, and serve straight from the skillet.