Heat a small Dutch oven that's coated in cooking spray over medium-high heat. Add the chicken sausage links and cook until all sides are golden brown, about 6-7 minutes. Remove from the heat and place on a cutting board with a tin foil tent. Add a teaspoon of olive oil in the same Dutch oven then add the grape tomato halves. Cook, flipping the tomatoes halfway through cooking, for about 2-3 minutes or until blistered. Remove from the pan and set aside. Add the remaining 2 teaspoons of olive oil to the pan then add the onion; cook, stirring often, for 3-4 minutes. Add the minced garlic, crushed fennel seeds, and crushed red pepper flakes, stirring constantly for 1 minute. Add the blended tomatoes then season with sea salt and freshly cracked pepper, to taste and a pinch of sugar if desired. Cover with a lid and turn the heat to low; simmer for 30 minutes.