Finely chop mushrooms and onions by hand or in a food processor. Heat 1 tablespoon oil in a large non-stick frying pan over high heat. Add the mushrooms and onions and cook, stirring for 8 minutes until all of the moisture has evaporated. Stir in herbs. Set aside to cool for 10 minutes. (mixture must be cool –you can not add warm mushrooms to cold mince mixture). Drain any excess moisture and transfer to a bowl. Wipe pan clean.
Add mince, egg and breadcrumbs to mushroom mixture. Mix until well combined. Shape tablespoons of mixture into balls with damp hands. Place onto a lined tray. Cover and chill for 1 hour until firm. (Or freeze for later use).
Heat oil in same pan over medium heat. Cook meatballs in 2 batches for 8-10 minutes, shaking pan often until browned and cooked through.
Spread baguette with mayonnaise. Top base with carrot, lettuce and cheese. Top with meatballs.