Melt butter in a skillet over medium heat and cook and stir 1 chopped onion in the melted butter until onion is translucent, about 5 minutes.
Place a large saucepan over medium-high heat and pour in pinto beans, kidney beans, pork and beans, tomato sauce, cooked onion, and 1/4 cup water, stirring to combine. Bring the mixture to a boil, reduce heat to low, and stir in 3 tablespoons sugar, chili seasoning mix, chili powder, and seasoned salt. Let the chili simmer while you finish the remaining steps, 10 to 20 minutes. Stir chili occasionally.
Mix 3 cups flour with baking powder, 6 tablespoons sugar, and salt in a large mixing bowl; stir in 2 cups of water or amount needed to make a workable dough. Spread 1 cup flour onto a work surface and scoop up about 1/4 cup of dough; drop it onto the flour and sprinkle surface with flour. Roll the dough into a flat round tortilla.
Heat oil in a deep-fryer or large skillet to 375 degrees F (190 degrees C).
Gently lay tortilla into the hot oil and deep fry until golden brown on both sides, 2 to 3 minutes per side. Dough will expand as it fries. Remove fry bread and drain on paper towels; continue to make and fry pieces of dough until all dough has been used.
Place lettuce, tomatoes, Cheddar cheese, 1 chopped onion, and sour cream into separate bowls. To serve, spoon a ladle of chili onto each fry bread and sprinkle with desired garnishes; top each serving with a dollop of sour cream.