Mumbai is India's melting pot, as well as its most cosmopolitan city, so it's no surprise that you can find nearly every regional Indian cuisine here, and some quality international food, too. You'll also find options ranging from casual to super chic.
Seafood from the Konkan coast—from Maharashtra south through Goa and all the way to Mangalore, in Karnataka—is a Mumbai specialty. The many seafood restaurants in Fort, from upscale Trishna to old-school Apoorva, have some of the best food in Mumbai. "Lunch home" is a typical Mumbai name for the slightly dingy seafood joints that bring in the crowds at lunchtime. North India is represented as well, with kebabs and tandoori. If you're looking for kebabs, head to restaurants specializing in Punjabi or Mughlai cuisine, or, if you have a fairly strong stomach, go to Khao Galli near Bhendi Bazaar; the late-night kebab snack option is Bade Miya behind the Taj Mahal Palace Hotel. Meat-heavy dishes from the North West Frontier (the area of undivided India that's partly in modern-day Pakistan) are also popular, and closely related to Mughlai food: check out Neel, at the Mahalaxmi race track in Central Mumbai, for upscale versions. On the other end of the spectrum are Gujarati vegetarian thalis—combination platters of various veggies and lentils, though the ones in Mumbai tend to be a bit oilier than those from elsewhere in India. Soam, at the top of Marine Drive, is a great upscale place for thalis, and they’re less oily here. You may also encounter some Jain food, which is also vegetarian but cooked without any root vegetables—and that includes onions and garlic. You'll find authentic South Indian vegetarian food—dosas (fried, crepelike pancakes), idlis (steamed rice cakes), wadas (also spelled vadas; savory fried, and often flavored, lentil-flour doughnuts), and simple, light thalis (combination platters)—all over the city.
There are many multicuisine restaurants around the city—usually fairly cheap, tacky joints that make good kebabs, decent Indo-Chinese food, and terrible continental food. (One rule of thumb for smaller or unknown places is that the further you depart from the Indian palate, the more likely you are to be burned with an unsavory dish.)
People eat late in India. Lunch is generally around 1-ish, and dinner is anytime between 8:30 pm and midnight—if you're meeting local friends, expect to eat around 9:30 or 10 pm. If you plan on eating at 7, reservations probably aren't necessary, and if you want to eat at 6, call ahead: your restaurant may not even be open for dinner yet. Locals generally dress for dinner. They aren't formal, but they are usually well turned out. Shorts and anything sloppy or grungy are only acceptable at cafés and dives.
It's worth noting, too, that if you're staying in South Bombay, there's no need to head out to the suburbs to eat. Nearby Lower Parel (roughly 25 minutes from Colaba Causeway by taxi), may lack tourist attractions, but it still manages pack in some of the more popular new eating and drinking hubs in the city.
The restaurant scene in Mumbai is always evolving. To keep an eye on the latest openings, as well as nightclub events, parties and food or drink festivals, check local papers, as well as the well-run local website Mumbai Boss (www.mumbaiboss.com).