Step 1

Preheat the oven to 375 degrees F (190 degrees C). Line a baking sheet with aluminum foil.

Step 2

Slowly and carefully cut butternut squash in half lengthwise, starting at the stem end. Scoop out the seeds with a spoon. Slice off about 1/8 inch from the bottom of each half to ensure they lay flat while baking. Transfer onto the prepared baking sheet.

Step 3

Place in the center of the preheated oven and roast until tender but not falling apart, 60 to 90 minutes. Remove from the oven and let sit until cool enough to handle. Increase oven temperature to 475 degrees F (245 degrees C).

Step 4

Use a spoon to scoop squash into a bowl, leaving about 1/2 inch attached to the skin so it won't collapse and fall apart later.

Step 5

Add salt, pepper, Dijon mustard, and egg to squash. Mix and mash with a spoon until smooth, or leave it a bit chunky, if desired. Stir in 1/4 cup green onions, ham, and diced Gruyere cheese; mix until combined.

Step 6

Scoop mixture back into the squash shells, distributing as evenly as possible and top with grated Gruyere.

Step 7

Bake in the center of the preheated oven until cheese is browned, about 15 minutes. Transfer to a cutting board and top with green onions. Slice and serve.