The torn romaine lettuce wilts just slightly in the soup and has a lighter, fresher, less earthy flavor than heartier greens like spinach or kale. Slice the carrot the same shape and thickness as the leek so you get a balance of both vegetables in every spoonful.
1 1/2 tablespoons canola oil, divided
1 pound skinless, boneless chicken thighs, cut into small pieces
Heat a large Dutch oven over medium-high heat. Add 1 1/2 teaspoons oil; swirl to coat. Add chicken; cook 6 minutes or until browned, stirring occasionally. Remove chicken from pan.
Add remaining 1 tablespoon oil to pan; swirl to coat. Add leek and carrot; sauté 5 minutes. Add stock and thyme; bring to a boil. Cover and cook 8 minutes or until carrot is almost tender. Stir in chicken, peas, pepper, and salt; cook 3 minutes. Remove thyme; discard. Remove pan from heat; stir in lettuce and parsley. Place 1 1/2 cups soup in each of 4 bowls; top each serving with 1 tablespoon cheese.