Finely chop onions and mushrooms by hand or in a food processor. Heat 1 tablespoon oil in a large non-stick frying pan over high heat. Add the onions and mushrooms 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 - do 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. Add carrot, lettuce and cheese. Top with meatballs.