Heat oil in a large Dutch oven over medium-high. Add sausage; cook about 6 minutes, stirring occasionally, until lightly browned. Add onion, celery, bell pepper, garlic, and jalapeno. Cook 5-6 minutes, stirring frequently, until vegetables are softened. Add rice and cook, stirring constantly, 30 seconds.

Add stock, tomatoes, salt, and cayenne pepper. Bring to a boil; cover, reduce heat to medium-low, and simmer until rice is just tender, 10 minutes. Add shrimp, cover, and cook until shrimp turn pink, about 4 minutes. Remove from heat. Stir in scallions and lemon juice. Garnish with additional scallions.