Wash and dry produce. Bring a large pot of salted water to a boil. 

Once boiling, add penne to pot. Cook, stirring occasionally, until slightly softer than al dente, 11-13 minutes. Drain, then rinse under cold water.

Meanwhile, trim and thinly slice scallions, separating whites from greens. Peel and mince or grate garlic. Halve grape tomatoes. Trim, peel, and grate carrot on the largest holes of a box grater. Drain olives, then roughly chop. Core, dessed, and dice bell pepper into ½-inch pieces. Zest and quarter lemon.

In a large bowl, combine pesto, mayonnaise, half the mustard, 2 tsp vinegar, ¼ tsp salt, ¼ tsp sugar, and a large drizzle of olive oil (for 4 servings, use all the mustard, 4 tsp vinegar, ½ tsp salt, ½ tsp sugar, and 2 large drizzles of olive oil).

Add scallion whites, garlic, lemon zest, and juice from half the lemon (juice from whole lemon for 4); stir to combine. 

Open and drain tuna; add to bowl with dressing. Stir until tuna is mostly broken up and evenly combined. Season with salt and pepper to taste.

Shake excess water from drained penne. 

To bowl with tuna mixture, add penne, tomatoes, carrot, olives, and bell pepper. Season with a big pinch of salt and pepper; stir to combine.

Divide tuna pasta salad between bowls and garnish with scallion greens. Serve.