Pre-heat oven to 400° F
Line a sheet pan with parchment and spray the parchment lightly with cooking spray.
Drizzle olive oil on both sides of the caps and season with a little salt and pepper
Bake for 15 to 20 minutes until the caps have softened.
Remove from the oven and keep warm until served
Mix all the ingredients for the oat burgers in a bowl, let rest 10 minutes, then form into patties.
Coat the bottom of a skillet with vegetable oil on medium heat and fry both sides of the burgers 3 to 4 minutes until nicely browned.
Mix the mayonnaise, mustard and ketchup in a small bowl.
Assemble the open-faced nutty oat burger on a plate with the lettuce, tomato, onion and pickles, and serve with the mayo sauce