Place onions into a bowl and pour juice from banana chillies until onions are covered.
Cover and place in fridge for 1 hour.
Place sweet peppers on a baking tray, cut sides down and drizzle with olive oil.
Place in a preheated oven at 400 and cook until pepper skins begin to blister, then remove from oven and allow to cool for 10 mins.
Place avocado, sour cream, yellow mustard and salt into blender and blitz until smooth.
If you have a squeeze bottle transfer sauce into it and place in fridge. If you don't just place the sauce in a small bowl in the fridge.
Peel away blistered skin from sweet peppers and rinse, then pat dry with paper towels.
Chop up sweet peppers and place in a bowl.
Drain onions and add to peppers, then add banana chilies.
Mix ingredients with a spoon or fork and set aside.
Grill hot dogs until warmed through then heat hot dog buns on grill while dogs are resting.
Spread some whole grain mustard on both sides of each bun.
Build hot dogs by placing one or two dogs in each bun, topping with relish, then topping with sauce.
If you do not have a squeeze bottle for the sauce, just add small dollops, 1/2 tsp at a time.
Serve immediately.