Preheat oven to 350°F. Remove stems from mushrooms and discard. Lay stem side down on parchment or silicone lined baking tray and brush with melted coconut oil. Cover with aluminum foil lightly and bake in preheated oven for 10 minutes. Remove and allow to cool slightly.
In a small bowl, mix the beef, 1 tbsp of the curry paste and dehydrated onions together until well blended. Using a patty form or your hands, form 2 oz patties (if you don't have a form or a scale, just divide the meat into 6 and form into patties).
Preheat a heavy cast iron pan on medium heat, fry patties for 2-3 minutes per side or until golden brown and well seared.
In a separate bowl, whisk the remaining curry paste with the coconut milk, cilantro, garlic, salt and lime until smooth.
Serve patties on mushroom caps as "buns" with sauce for dipping or pour sauce over patties before topping with second cap.