Put the onions in a heavy-based pan with the olive oil, bay and cinnamon. Cook over a medium heat, stirring the onions, until they are starting to turn golden, then add the garlic and cook for a further two minutes.
Add a splash of water, cover the pan, turn the heat right down and leave until the onions are almost caramelised, about 35 minutes. Check on them every so often to ensure they’re not getting too dry.
When the onions are totally soft, remove the lid, season and boil until any excess liquid is evaporated. (If they haven’t caramelised very well you could add a teaspoon of sugar at this stage, but balance it by adding a good squeeze of lemon.)
Cook the tagliatelle until al dente, about 15 minutes.
When the pasta is almost ready add the yogurt and milk or buttermilk to the onions and heat through but don’t boil. Drain the pasta and toss it into the pot with the onions, adding the dill.
Very quickly melt the butter in a small saucepan and add the cayenne. Cook for about 20 seconds.
Serve the pasta with the spiced butter drizzled on top and offer finely crumbled feta on the side.