Miami's temperature is approaching 90 degrees. South Beach's streets are clammy, damp from the midsummer mist. You are hungry, but you don't want steak frites, lasagna Bolognese, or thick vegetable curries with sticky rice. You want Khong River House's boat noodles ($18) — a robust, auburn noodle-and-broth soup enriched with fish sauce, beef blood, fried garlic, and chili vinegar. You want Khong's Vietnamese-style crispy prawns ($31) with spring onions and shallots or Thai tofu salad ($13) with deep-fried bits of soybean curd and vibrant vegetables. Owned by 50 Eggs Inc., the folks behind Yardbird Southern Table & Bar, the restaurant is an oasis of intrepid flavors and startlingly spicy cuisine. Its cooking is based on foods found across the northern Mekong River, particularly in Laos, Burma, Vietnam, and Thailand. Many dishes were previously hard to find around town. Now, when it's hot out, Khong's jolts of piquancy and bursts of freshness feel so right.