For the lavender hollandaise, first finely chop the spring onion and put it in a pot with about 125 ml of water. Add the peppercorns, lavender flowers and vinegar and boil down until only about 50 ml of liquid remains. Remove from heat, strain and let cool briefly.
Now, in a bowl, beat the egg yolks over steam with about 2 tablespoons of lavender essence, a pinch of salt and a squeeze of lemon juice until foamy. At the same time, in a saucepan, slowly melt the butter without letting it brown. Once the yolk mixture has thickened, whisk in the melted butter, first drop by drop, then a little more quickly, until the sauce binds nicely and becomes creamy. (If the sauce still curdles, stir in a tablespoon of cold water and continue beating).
Dress the warm lavender hollandaise, but do not let it boil again (otherwise it will curdle!). Garnish with the lavender flowers if desired.