Left fire radical - the indexing component, signalling cooking with heat. 炖 specifically means slow, simmering fire under a closed pot. Same radical anchors a kitchen family: 炒 (stir-fry), 烤 (roast), 烧 (burn/cook), 煎 (pan-fry).
Right 屯 supplies the sound (tún → dùn, mild d/t drift) and a faint semantic echo: 屯 means to gather and hold in place - exactly what stewing does, holding ingredients together over slow heat. Phonetic family includes 钝 (dull), 顿 (pause), 沌 (chaos).
