Remove the frozen veg from the freezer, and leave to defrost for around 15 minutes.
Add the defrosted veg, coriander (stalks and all), flour, cumin and cayenne to a food processor. Season well, then blitz to a rough paste. Add the sesame and sunflower seeds, then pulse to combine.
Divide and shape the mixture into 4 equal-sized patties, roughly 2cm thick. Add the patties to a lightly flour-dusted tray, turning them over in the flour to coat. Place in the freezer for 10 minutes to firm up, then pop in the fridge while you make the vegan mayo.
Trim, roughly chop and add the spring onions and chilli to a blender. Peel and add the garlic, along with the chickpeas, tomato paste, mustard, brandy, lemon juice and a good pinch of sea salt and black pepper. Blitz well to combine then, with the liquidiser still running, slowly pour in 4 to 5 tablespoons of extra virgin olive oil until smooth and the same consistency as mayonnaise.
To make the onion rings, peel and slice the onion horizontally into rounds, roughly 2cm wide. Separate them into rings, then place into a bowl with the vinegar and a pinch of salt. Leave for 5 minutes.
Meanwhile, combine the flour and baking powder in a bowl, pour in the ale and whisk gently until smooth, thick and coating the back of a spoon nicely.
Place a medium, deep pan over a high heat, pour in the sunflower or sunseed oil and allow to heat up. To test if the oil is hot enough, drop a piece of bread into the pan – if the bread floats to the surface, sizzles and turns golden, it’s about right.
Drain the onions, dust a handful of them with flour until lightly coated, then dip into the batter. Remove with a slotted spoon and allow any excess batter to drip off, then carefully lower into the hot oil. Cook for around 1 minute, or until golden and crisp, turning halfway with the slotted spoon. Transfer to a double layer of kitchen paper, then repeat with the remaining onion.
Preheat the oven to 160ºC/325ºF/gas 3.
Heat a splash of olive oil in a large frying pan over a medium heat, add the burgers and cook for 10 to 12 minutes, or until cooked through and golden, turning halfway and pressing down with a fish slice to get them nice and crispy. Transfer to a baking tray, then pop in the oven while you prepare the remaining ingredients.
Finely shred the lettuce. Add to a large bowl with half of the vegan mayo (keep the rest in the fridge for up to two days and use as a tasty dip or in sandwiches). Mix well.
Slice the tomatoes and place on a plate lined with kitchen paper. Sprinkle over a little salt and pick over the basil leaves. Top with another piece of kitchen paper and pat lightly to remove any excess water. Slice the gherkins lengthways, then halve and destone the avocado. Scoop out the flesh and cut into slices, then squeeze over the lime juice.
Halve and toast the burger buns on a hot griddle pan or under the grill. Once ready, pile the lettuce and sliced tomatoes onto the burger bun bases. Pop a burger on top, snip over the cress, then layer over the gherkins and crispy onion rings.
Add a good squeeze of ketchup to the burger tops, place them on top, squeezing down gently, then tuck in.