Wash the clams well in cold water, and drain.
Heat the olive oil, add the shallots and thyme, and cook until fragrant.
Add the clams, lemon and wine, cover and cook until the clams have opened.
Tip into a colander placed over a bowl.
Strain the liquid through a fat filter; this will remove any sand that is present. Boil to reduce by 75%.
Pick out the clam meat from the shells and finely shred.
Macerate the shallots in the vinegar for 10 minutes.
Add the mustard and whisk in the oil followed by the cooled clam juice and shredded clams.
Finish just prior to serving with the herbs and finely diced tomato.