Grill the mushroom and onion over charcoal until the mushroom has released its moisture and the onion is charred and soft. The mushroom should take twice the amount of time as the onion will need on a hot grill. Chop the onion and mushroom into pieces.
Meanwhile, combine the lentils and bay leaf and cover with water in a small pot. Bring to a boil and simmer for 12 minutes.
Cook spaghetti noodles according to package directions. Drain.
Mix together cooked lentils, grilled mushroom and onion, garlic, marinara sauce, balsamic vinegar, and salt and pepper.
Bring to a boil to heat through or if you have time, let it simmer for 10-15 minutes to let the flavors meld.
Serve over cooked spaghetti.
Notes
The time will be greatly reduced if you grill the mushroom and onion in advance as a part of meal prep.
Recipe by Alison's Allspice at https://www.alisonsallspice.com/grilled-portabella-and-lentil-spaghetti/