Place eggs into the simmering basket and submerge in water inside the Thermomix bowl.
Cook 15 minutes/100°C/speed 4. Cool and peel eggs, and slice into discs.
Place onion into the clean, dry Thermomix bowl and chop 2 seconds/speed 5.
Add remaining ingredients except the cheese and eggs, and stir through 4 seconds/speed 4.
Butter the bread and place under a hot grill to lightly toast on one side. Remove.
Spread the tuna mix onto the untoasted side of the bread, top with sliced eggs, cheese and return to the grill/broiler until golden and bubbling. Finish with a few thyme leaves to serve.