Preheat the oven to 180C (160C for fan ovens)/350F. Cut the cabbage into 8 wedges right through the core, then remove most of the core but keep the leaves attached. Arrange the wedges on a baking pan, then brush with the olive oil on both sides and season with salt, roast in the preheated oven for 15 minutes or until the cabbage is softened and the outer leaves are getting slightly charred. Keep an eye on it to avoid burning.
Meanwhile melt 1 tbsp of butter in a pan and add the sliced leeks, sauté over low heat for 15 minutes until soft, then add the minced garlic and cook for 30 seconds more, now deglaze with the white wine and let the liquid reduce to almost nothing.
Add 2 tbsp of butter and melt, then mix in the flour until the mixture makes a loose paste, continue cooking while stirring for a minute, then slowing start mixing in the milk stirring the whole time to avoid lumps, bring to a simmer and cook for a few minutes until it thickens, then fold in half of the grated cheese and allow it to melt into the sauce, taste and add salt. Remove 250ml/1 cup of the cheese sauce from the pan and reserve for later.
Take the cabbage out of the oven and arrange on top of the sauce, then pour the reserved sauce over the cabbage wedges and sprinkle with the remaining grated cheese, cover with tin foil and put back in the oven for 20 minutes or until the sauce is bubbly and the cheese is melted, then uncover and cook for 10 minutes longer or until the cheese is starting to colour.