Clean the mushrooms and cut them in half. Cook them in a pan without oil on medium to high heat until they are soft. Set aside and let them cool off a bit.
Peel and cut the red onion in rings. Slowly caramelize the red onion in a pan while adding bits of water when it starts to stick to the pan.
Transfer the cooked mushrooms and about 1/3 of the caramelized onions to the food processor. Add sunflower seeds, oats, breadcrumbs, fresh dill, and salt. Process until combined. You should be able to form 6 patties. If the mixture is too crumbly add tiny splashes of water, if it's too sticky add more breadcrumbs.
Place the mushroom patties on a baking tray lined with parchment paper. Spray or brush them with oil. Bake the patties in the oven for 20-25 minutes, flipping them once halfway through. They should be slightly browned.
Assemble your burgers: Slice the pretzel buns open, add fresh arugula, the mushroom patty, spread mango chutney on top and add leftover caramelized onions. Enjoy!
These patties are great for freezing! Layer them between small sheets of parchment paper to prevent them from sticking together. Let them thaw after taking them out of the freezer and reheat in a pan with a little bit of oil.