Melt the butter in the Dutch oven. Char the bell pepper, zucchini, eggplant, and sweet onion on the grill or over the fire until blackened, let them cool, then remove the blackened skin and any seeds if necessary. Finely chop everything and mix together.
Combine the cream and ajvar, then season with 3–4 tablespoons of fries seasoning.
In the Dutch oven, layer 2–3 layers of potatoes at a time, seasoning each with salt and pepper (and chili powder to taste), then top with a layer of onion rings and some cheddar cheese. Repeat until the pot is full, then pour the cream mixture over everything.
Close the Dutch oven and slowly heat it over the fire or with coals, being careful not to burn the bottom or top. (Rule of thirds: pot diameter in inches equals the number of briquettes. Place one third underneath and two thirds on the lid.) Add more coals as needed until the potatoes are tender and the top is golden brown. Add a little more cheddar and the feta cheese, then briefly broil with higher top heat until gratinated.
