1. Put ½ the grated cheese onto the bottom of the tart case.
2. Put the egg yolks, cream and the rest of the cheese into a bowl with some salt and pepper and mix well. Pile the mixture into the tart case along with the other cheese.
3. Put the tart into the oven and bake for 15-20 minutes, or until just set (it may be a bit wobbly when shaken).

To make the onions

1. Heat a pot and add the olive oil and butter. Let it melt, then add the onions and cook on low heat until starting to wilt and color, stirring frequently. (The bottom may catch a little. Just don’t scrape it.)
2. When the onions are golden in color, add the marmalade and the white balsamic vinegar. Continue to cook until the onions become a deeper golden color and look kind of glossy.
3. Add a tablespoon of sweet balsamic to finish.

To make the walnuts

1. Heat a non-stick frying pan and add the walnuts. Toss over a medium heat until starting to color.
2. Once they are lightly colored, turn off the heat and sprinkle over the sugar, tossing to coat. The sugar will melt with the residual heat and coat the walnuts. Cool on a plate.
3. Serve the tart with a little extra balsamic (sweet or aged).