Preheat oven to 400F.
Cut end off squash, and slice lengthwise. Scoop out seeds until halves are hollow.
Brush olive oil on cut rims and place cut side down on parchment-lined baking tray.
Place in oven and cook for 45 minutes to an hour, until fork tender.
While the squash is cooking, prep the meat stuffing.
In a large pot, cook ground beef on medium-high heat until brown (about 5 to 8 minutes).
Reduce heat to medium.
Add onion, garlic, chili powder and salt. Mix well and cook for 2 minutes.
Add mushrooms, lentils and tomatoes. Cook for 5 minutes.
Take squash out of oven once cooked.
Spoon beef mixture into squash “bowls” and top with cheese. Sprinkle on chives if using.
Broil in oven until the cheese is melted to your liking.