burn, roast
炮 centers on applying direct heat to food: burning or roasting, typically over a fire.
炮 combines 火 (fire) with 包, which likely contributes the sound and the idea of wrapping or enclosing, suggesting food wrapped and roasted over fire.
Fire 火 wraps around food like a package 包, roasting it until it's perfectly cooked.
For ホウ, imagine a whole pig being roasted over a fire: whole -> ホウ, and the fire wraps around it like a package.