Combine tuna, eggs, shallot, lemon zest, salt, pepper, mayonnaise, mustard, 1 tablespoon dill, and ½ cup breadcrumbs, adding the remaining breadcrumbs if the mixture is too wet to hold together.
Form the tuna mixture into 8 patties ½-inch thick, then put them in the refrigerator to chill.
Squeeze all of the moisture out of the grated cucumber using a paper towel or cheesecloth.
Mix the cucumber with the yogurt, garlic, lemon juice, 1 tablespoon olive oil, and remaining dill. Season the sauce with salt and pepper to taste.
If the sauce is too thick, thin it with additional oil or water.
Heat the remaining oil in a frying pan and remove the tuna patties from the fridge.
Fry the tuna patties for 3-4 minutes per side until crisp and golden brown, working in batches to avoid crowding the pan.
Drain the tuna patties on a wire rack or paper towel.
Top the tuna patties with the sauce before eating, and garnish with dill if desired.