Preheat oven to 180°C.
Make the patties. Peel and finely dice the shallots. Place a frying pan over a medium heat and add the olive oil. Once warm, add the shallots and a pinch of salt. Mix well and cook for three to four minutes until soft. Add the brown sugar to the pan and cook for a further three to four minutes until the shallots have caramelised. Once cooked, take the pan off the heat, spoon the shallots into a bowl and leave to cool for a few minutes. To the bowl, add the plant-based mince, breadcrumbs, BBQ sauce and season with salt and pepper. Mix until well combined.
Shape the patties. Lightly oil your hands and shape the mixture into four plump patties or eight smaller patties. Set the patties out on a lined baking tray and ‘smash’ them with the base of a wide, flat-bottomed glass – if needed you can shape the sides neatly using your hands. Cook in the oven for 25 minutes.
Prepare the remaining ingredients. Halve the brioche buns. Drizzle the cut sides with the olive oil and toast in the frying pan over a high heat until golden brown. Remove the core of the lettuce and shred the leaves. Thinly slice the tomato. In a small bowl, mix together the plant-based mayo and BBQ sauce until smooth.
Finish the burgers. After 25 minutes, remove the tray from the oven. Place the plant-based cheese slices on top of the burger patties and return the tray to the oven for one minute to melt the cheese.
Time to serve. Cover one half of the burger buns with the BBQ mayo. Take the patties out of the oven and transfer to the burger buns. Dress the burgers with sliced lettuce, tomato and crispy fried onions. Close the burgers and serve immediately.