Place all ingredients in a small food processor and process until smooth.
Season with sea salt. Set aside.
Use a small sharp knife to cut the ends off each tomato, then slice in half crossways to produce pieces that can stand upright. Arrange on a serving plate.
Heat a small non-stick frying pan on medium and cook peppercorns for 1 minute until fragrant.
Transfer to a mortar and pestle and pound until coarsely chopped.
Combine bocconcini and a spoon of pesto in a small bowl, tossing until evenly coated.
Spoon a small amount on the cut surface of each tomato. Sprinkle with peppercorns and drizzle with remaining pesto.