For a stronger tomato flavour, you can, after adding the mustard and before the tomatoes, add a layer of pizza sauce.
Remarks
The tomatoes depending on of their origin and ripeness could give off a lot of juice, which can be a bit annoying as it makes the pastry soggy. To try to reduce this there are several methods:
Do not salt before cooking, because it's the salt which brings out the juice. Instead salt salt just after bringing out of the oven.
Drain off as most of the juice partway through cooking, but it's a slightly delicate operation. For this Francoise uses a large plastic ex-medical syringe.
Salt the the tomatoes and leave the juice to drain off before using (see above).