Remove seeds and pulp from the pumpkin. Microwave the pumpkin for 5 minutes at 600W until softened. Slice the onion thinly and separate the shimeji mushrooms into small clusters.

Trim excess fat from the chicken thigh and cut into bite-sized pieces. Season with salt and pepper.

Heat olive oil in a pan over medium heat. Cook the chicken for 5 minutes until fully cooked. Add onion and sauté until softened, then add shimeji mushrooms and stir until coated with oil.

Sprinkle flour into the pan and stir until no raw flour remains. Gradually add milk while stirring, cooking until thickened. Add salt, pepper, and butter, then mix well. Cool the sauce in the refrigerator for 30 minutes.

For decoration, layer sliced cheese and cheddar cheese alternately. Use cookie cutters to create ghost and jack-o'-lantern shapes. Cut seaweed into facial features and attach to cheese shapes.

Place the pumpkin on a baking tray lined with parchment paper. Fill the pumpkin with white sauce and top with pizza cheese. Bake at 230°C for 8 minutes until cheese melts.