Preheat oven to 375°F (190°C). Place garlic and squash (cut-side down) on parchment paper-lined baking sheet. Brush with 1 tsp (5 mL) of olive oil and season with salt and pepper. Roast until tender, about 40 min. Squeeze out garlic and scoop out squash, discarding skins. With a fork, mash garlic and squash together.
Heat 2 tsp (10 mL) olive oil in a heavy-bottomed, medium-size saucepan over medium heat. Add onion and sauté, stirring until translucent, about 3 min. Add the rice, stirring to coat in oil until lightly toasted, about 2 min. Then, add squash mixture.
Add wine and stir until almost fully absorbed. Then, add chicken broth one cup (250 mL) at a time. Stir often, letting rice almost fully absorb broth before adding another cup.
Continue cooking until rice is creamy, cooked through but still firm to the bite, about 20 min. Stir in Parmesan cheese and pepper. Serve with a drizzle of the remaining olive oil and a shaving of fresh Parmesan if desired.