Drain the tuna of oil and put it in a blender with eggs, mayonnaise, crème fraîche, dill, and herbs. Blend until you get a smooth and fine mousse. Season with salt and pepper.
Soak gelatin in cold water, wring it free of water, and put in a small saucepan with lemon juice. Melt the gelatin and lemon together over low heat.
Put a spoonful or two of the tuna mousse into the gelatin/lemon mixture and stir together. This lowers the temperature. The gelatin should ideally be no warmer than finger-warm before the next step. If you have the time and patience, you can also just let the gelatin cool until it is finger-warm.
Now pour the gelatin/tuna mixture back into the blender and blend a few times to distribute it well in the tuna mousse.
Line a springform pan, about 15 cm, with cake plastic around the edges and parchment paper on the bottom. Spread the tuna mousse in the form and refrigerate until the mousse has set. It takes about an hour, but if you put a plate or lid over the form so the mousse doesn't dry out, it can easily be made the day before, so it's just ready to be decorated with shrimp, etc., when you need it.
When the mousse has set, top it with shrimp, tomato, cucumber, and lemon. Brush the edge of the mousse with a little water and press the chopped herbs on with a knife.
Serve with a good piece of bread.