To prepare the grits, combine the chicken stock, milk and garlic powder in a medium pot over medium-high heat. Bring the liquids to a boil and then stir in the grits. Reduce the heat to medium low and cook, stirring frequently, until the grits are tender and have absorbed the liquid, 6 to 8 minutes. Stir in the cheese and season the grits with salt and pepper. Reserve warm.
While the grits are cooking, place a large skillet over medium heat with the butter. Add the onion, bell pepper and garlic to the pan and cook, stirring occasionally, until tender, 3 to 4 minutes. Add the shrimp, tomatoes and juice of the lemon to the pan and simmer until the shrimp are cooked through, 3 to 4 minutes. Stir in the zest of the lemon and tarragon and season the shrimp with salt and pepper.
To serve, spoon the shrimp and pan sauce over the grits.