Place quartered tomatoes in 8–inch (2 L) square glass baking dish. Drizzle with 1 tbsp (15 mL) of the oil. Arrange tomatoes skin side down. Roast for 5 minutes. Reduce oven temperature to 300°F (150°C). Roast for 1 hour.
In small bowl, stir together remaining 2 tbsp (25 mL) oil and basil. Set aside.
Arrange mixed greens on large platter. Place warm tomatoes on top of greens. Scatter with mini bocconcini balls. Drizzle with basil mixture. Sprinkle with sea salt. Serve while warm.