Peel and slice the red onion and fry in a pan with butter, stirring regularly. When it is melting and has turned golden, stop cooking and set aside.
Peel and chop the garlic and shallot. In a food processor, blend the drained tuna, cream cheese, garlic and shallot.
Pour in the olive oil and lemon juice, then season with salt and pepper. Blend to obtain a smooth preparation but with some pieces.
Place a generous layer of thoinade on top of a gluten-free Sigdal oatmeal crunch. Add a few slices of cucumber and cooked red onion and sprinkle with a few sprigs of fresh dill.