Mix lemon juice, fajita seasoning and 1 tbs oil together in a large bowl. Add mushrooms, toss to coat. Cover and set aside 10 minutes. Cook rice following absorption method on packet.
Heat a large non-stick frying pan over high heat, add mushrooms and cook 4-5 minutes until golden and tender. Transfer to a bowl. Add remaining 1 tbs oil and onion to the pan, reduce heat to medium and cook, stirring often, 3-4 minutes until softened. Stir in the mushrooms, remove from heat.
Combine coriander, chillies and avocado in a bowl. Spread chipotle mayonnaise over each tortilla. Top each with a spoonful of rice, mushrooms and avocado mixture. Fold into quarters and serve with lemon wedges.