Heat the oven to 425°F (215°C). Place an oven rack on the first/top position in the oven, close to the heating element. Place a large baking sheet in the oven to preheat.
In a large mixing bowl (large enough to fit the chicken, potatoes, onion and marinade), whisk together the mayonnaise, miso paste, and maple syrup. Reserve ⅓ cup of mixture for serving.
To the bowl, add the paprika, garlic, bouillon, and whisk to combine. Add the onion, potatoes, and chicken thighs. Stir until evenly coated.
Carefully remove the hot pan from the oven. Dump the chicken, potatoes and onion in the pan and arrange in an even layer, lifting the chicken thighs on top so they lay on a bed of potatoes and onion. Season with a light sprinkle of salt on top.
Scrape any leftover marinade in the pan. Roast for 45 to 55 minutes, until the chicken has a sticky crust on top and the potatoes are tender. If anything browns too fast, loosely cover the pan with aluminum foil and continue cooking.
Sprinkle the thinly sliced pepper on top and the reserved miso maple mayonnaise on the side.
