Finely chop stalks. Place them in a frying pan with the chopped onion, garlic and ½ tbsp oil. Sauté gently until softened, then add the paprika and thyme, and cook for another minute. Tip into a bowl with the breadcrumbs, season and mix well.
Heat oven to 300°F. Brush the Portobello mushrooms with olive oil and heat a griddle pan. Sauté the mushrooms on both sides until golden. Divide the breadcrumb mixture between 2 of the mushrooms and put the other mushrooms on top.
Place in a roasting tin and bake for 5 minutes. Open the buns and add them to the oven for another 5 minutes with the mushrooms.
Sandwich the double mushrooms between the toasted buns with some lettuce, sliced red onion, wholegrain mustard and 2 Violife Cheddar Slices in each burger. Serve with gherkins and vegetable crisps.