Preheat the oven to 200C/400F/Gas 6.
For the stuffed tomatoes, heat half the oil in a wok or frying pan over a medium heat and fry the garlic for 1-2 minutes.
Add the spinach and chilli flakes, season with salt and freshly ground black pepper and cook, stirring, until the spinach has wilted. Remove from the heat, leave to cool slightly then squeeze the excess moisture out of the spinach and roughly chop.
Spread the mustard inside the tomatoes, then fill with the chopped spinach mixture.
Place the tomatoes onto a baking sheet, drizzle over the remaining olive oil and bake in the oven for 6-8 minutes, or until softened.
Meanwhile, for the parsley pesto, place all the pesto ingredients into a mini food processor and blend until smooth.
To serve, place the tomatoes onto a plate, drizzle over the pesto and sprinkle over the chives.