Cut chicken into bite-size strips, about 1/4-inch wide. Heat 1 tablespoon oil in a large nonstick frying pan set over medium-high heat. Add about half of chicken ad stir-fry until almost cooked, from 3-4 minutes. Remove to a plate. Repeat with remaining chicken, add more oil if needed. Remove to plate.
Add remaining oil to pan, then mushroom, ginger, jalapeno, pepper, garlic, and chili flakes. Stir often until lightly browned, about 5 minutes. Stir in broth, fish sauce and sugar. When boiling, add chicken and and any juices that have collected on plate. Cook uncovered, and stirring often, just until chicken is done as you like it, or feels springy to the touch, about 2 minutes. Sti in onion and basil. Taste and add a little more fish sauce if you like.