Heat the olive oil in a large saucepan over medium heat. Add the garlic and cook, stirring frequently, until fragrant but not browned, about 1 minute. Add the tomato paste, red pepper flakes, Harissa and 1 teaspoon each salt and black pepper and cook, stirring constantly, until the mixture darkens, about 1 minute.
Add the cherry tomatoes to the saucepan, mashing a few with a wooden spoon. Cook, stirring occasionally, until the tomatoes split and soften, 8 to 10 minutes. Stir in the vinegar and 1 1/2 cups water and bring to a simmer over medium-high heat. Cook until slightly thickened, 5 to 7 minutes.
Add the salmon to the saucepan in a single layer. Reduce the heat to medium, cover and cook until the salmon is opaque throughout and flakes easily when a paring knife is inserted between the layers, 7 to 9 minutes. Serve the salmon in shallow bowls with the tomato sauce ladled over.