Heat two tablespoons of the oil in a pan over a low heat and gently fry the onion and chopped garlic for 3-4 minutes, or until softened. Add the chilli and tomatoes and cook for a further 4-5 minutes, then stir in the basil.
Heat a chargrill pan, drizzle the bread with the remaining oil and toast the bread for 2-3 minutes on each side, or until golden-brown and crisp all over. Rub the toasted bread with the whole garlic clove.
To serve, arrange the toasted bread on a plate and spoon over the tomato mixture. Top each bruschetta with a dollop of yoghurt and serve immediately.