The cooking sense is 炸 (2nd tone, rising), never zhà. Menus all over China write 炸鸡, 炸薯条, 炸豆腐 — getting the tone right marks you as someone who actually orders food in Chinese.
culture
The blanching sense (炸 a vegetable for a few seconds in boiling water) is closer to modern 焯 in everyday speech; you'll see it mostly in older cookbooks. The dominant modern meaning is deep-frying in oil — the technique behind 油炸糕 and 炸酱面.
Left 火 is the indexing fire radical in its full four-stroke form. For the deep-fry sense the fire under the wok is literal — oil hot enough to crisp food. Cooking-radical neighbors: 煎 pan-fry, 煮 boil, 烤 roast.
Right 乍 supplies the sound, though for this reading the tone has dropped from 4th to 2nd (乍 → zhá) — a small drift inside the same phonetic family that also gives 诈 and 榨. Older dictionaries filed this sense under 煠, later merged into 炸.
