spicy thai chicken soup recipe

3 lemongrass stalks, bottom two-thirds of tender inner bulbs only, thinly sliced 4 cloves garlic, chopped 1 (4 inch) piece fresh ginger root, chopped 4 cups chicken broth 1 tablespoon vegetable oil 2 ½ pounds skinless, boneless chicken thighs, cut into chunks 12 ounces fresh white mushrooms, quartered 3 tablespoons fish sauce 2 teaspoons red curry paste 1 lime, juiced