In a large bowl, whisk together the Dijon mustard, lemon juice and olive oil.
In the same bowl, add in the cans of tuna with their olive oil and the cannellini beans. Break the tuna into large chunks using a fork.
Add the shallots, capers, dill, parsley, salt and pepper and mix well. Allow the mixture to marinate for at least 15 minutes.
While the tuna is marinating, bring a medium saucepan of salted water to a boil.
Steam the sugar snaps and mangetout for 3 minutes in the boiling water and drain immediately.
Add the sugar snaps and mangetout to the tuna and lightly toss. Taste for seasoning.
Toast the country bread slices and rub with the garlic cloves.
Generously spoon the marinated tuna and cannellini bean mixture over the toast and drizzle any extra marinade over the top.
Garnish with dill and parsley leaves.