In a large mixing bowl, stir together breadcrumbs and milk, then add all remaining burger ingredients. Use your clean hands to mix and combine the ingredients together well. Shape into 4-5 patties (about 2cm thick) and set aside on a plate in the fridge for 5-10 minutes to firm up a little.
Finely dice roast capsicum. Place crushed tomatoes in a sieve and use the back of a spoon to press down on them to squeeze out most of the excess liquid. Mix with diced roast capsicum, basil leaves and chilli flakes (if using) and season to taste with salt and pepper.
Heat a good drizzle of olive oil in a large fry pan or on a preheated BBQ hotplate on medium heat. Cook beef burgers and then whole Portobello mushrooms (while the beef burgers rest) for 3-4 minutes on each side, until beef burgers are well browned and cooked through, and mushrooms are soft. Use a fish slice to press down on the patties and mushrooms while they are cooking.
Mix mayonnaise, lemon juice and paprika together and season to taste with a little salt.
Cut burger buns in half and warm in the oven or in the pan or on BBQ.
To serve, top each burger bun with some smoky mayo, rocket leaves, a burger patty, mushroom, and slices of tomato and onion. Top with salsa, a little more mayo and top burger bun.