Put the basil leaves, lemon juice, and olive oil in a small food processor to prepare the dressing. Once the mixture is nearly smooth and the leaves are finely minced, process. Salt and pepper should be used to taste.
Depending on their size, cut the tomatoes into halves or quarters. In a bowl, mix the tomatoes, spinach leaves, and spring onions.
Just before serving, dress the salad well by drizzling it with only half the dressing. Serve the remaining dressing separately.