Preheat oven to 400°F (200°C). Season cut side of squash half with salt and pepper.
Arrange, cut side down, on a baking sheet lined with aluminum foil. Cover squash with more foil and bake in centre of oven for 25 minutes.
Meanwhile, mix together butter, maple syrup, garam masala and salt and pepper in a small saucepan. Heat the pan on low heat until butter has melted; immediately remove from heat.
After 25 minutes, remove squash and flip it so that the cut side faces up. Brush with half of the maple syrup mixture and keep remaining mixture warm by covering the pan with a lid. Return squash to oven and bake until squash is tender, about 15 to 20 minutes.
Meanwhile, toast seeds in a small, dry non-stick skillet over medium heat until fragrant, about 2 or 3 minutes. Remove from pan, season with salt and pepper and let cool. To serve, cut squash into 1-inch slices crosswise, drizzle with remaining maple syrup mixture and garnish with toasted squash seeds.