1.Preheat oven to 240°C/475°F.
2.For the tomato concasse, heat oil in a medium saucepan; cook shallot and garlic, stirring, until shallot softens. Add tomato and vinegar; cook, covered, over low heat, 15 minutes. Uncover; simmer, stirring occasionally, about 30 minutes or until mixture thickens slightly.
3.Meanwhile, quarter capsicum; discard seeds and membranes. Roast, skin-side up, in oven until skin blisters and blackens. Cover capsicum in plastic wrap or paper for 5 minutes; peel away skin, then slice capsicum thinly.
4.Reduce oven temperature to 180°C/350°F.
5.Place lamb, cut-side up, on board; rub garlic into lamb. Place capsicum, spinach and basil down centre of lamb; roll tightly, securing at 2cm (¾-inch) intervals with kitchen string. Rub oil all over lamb roll; season.
6.Place lamb on an oiled wire rack over a large shallow baking dish; roast, uncovered, about 1 hour or until lamb is browned and cooked as desired. Cover to keep warm.
7.Serve lamb with tomato concasse.