Nepali cuisine is honest, filling, and not particularly adventurous — but it is good. Dal bhat tarkari (lentil soup, rice, and vegetable curry) is the national dish and the fuel that keeps everyone going, from farmers to trekking porters. Most Nepalis eat it twice a day. The rice is plain, the dal is thin, the tarkari comes with whatever vegetables are in season — but the combination is perfectly balanced and will serve you well on the trail.
Momos are the other essential. These steamed or fried dumplings — filled with spiced minced meat or vegetables — are sold everywhere from street stalls to proper restaurants. The spicy tomato chutney they come with is frequently the best sauce you'll eat in the country. Buffalo momo is traditional; chicken is the safer choice in tourist areas.
Tibetan influence shows up throughout the mountain regions. Thukpa (noodle soup) and tsampa (roasted barley flour) are trekking staples at altitude, where the heavy carbs make sense. Sherpa stew is a thick, warming potato and vegetable soup that appears on most teahouse menus from Namche Bazar upward.
The Newars of the Kathmandu Valley have the most complex cuisine. Chatamari (a thin rice-flour crepe topped with meat, egg, and spices, sometimes called "Nepali pizza") is genuinely delicious. Newari feasts, served during festivals, can involve dozens of small dishes including boiled eggs, dried meat, beaten rice, and various fermented vegetables. If you get an invitation, accept it.
Sel roti — a crispy fried ring-shaped bread made from rice flour — is a street snack and a festival food, particularly during Dashain and Tihar. Sweet, crunchy, and hard to stop eating.
In Kathmandu, the restaurant scene is surprisingly good. Thamel is full of tourist restaurants serving everything from pizza to Korean, but the Newari restaurants in the old quarters and around Boudhanath tend to have better food. Try to eat at least once at a local bhansa ghar (home restaurant) if you can find one.
Drinking water is not safe from the tap anywhere in Nepal. Filter or purify it, or buy bottled water. On treks, boiled water from teahouses is fine; avoid ice in areas without reliable electricity. Nepali tea (chiya) — brewed strong with milk and sugar, often with cardamom — is excellent and safe wherever you drink it.