Preheat oven to 200C/180C fan-forced. Grease a 6cm-deep, 22cm x 32cm (8-cup-capacity) baking dish. Cook pasta following packet directions. Drain. Return to pan. Meanwhile, heat oil in a large deep frying pan over medium-high heat. Add onion. Cook, stirring, for 5 to 7 minutes or until light golden. Add garlic. Cook, stirring, for 1 minute or until fragrant. Add 1/2 cup water, sauce and pesto. Bring to boil. Reduce heat to medium-low. Simmer, stirring occasionally, for 10 minutes or until slightly thick.