Trim and discard tough top and root end of 1 stalk (12 in.) fresh lemon grass. Remove and discard tough outer layers. Thinly slice tender inner part, then whirl in a food processor until finely chopped. Add 2 cups sliced shallots, 3 tablespoons sliced garlic, 1/2 cup sliced fresh red or green jalapeños, 3 tablespoons sliced peeled fresh galangal (optional), 2 tablespoons thinly sliced peeled fresh ginger, 1 1/2 teaspoons coriander seed, 1 teaspoon ground dried turmeric, and 6 salted roasted macadamia nuts; whirl until finely ground. Put spice paste and 2 tablespoons salad oil in an 8- to 10-inch frying pan over high heat. Stir until paste is slightly darker and drier, 6 to 8 minutes. Use hot or cool. If making up to 3 days ahead, cover and chill; freeze to store longer.