1. Slice the spring onions into 1/2-inch (2cm) pieces. Melt the butter in a skillet and cook over medium heat, seasoning with a bit of salt and pepper, until tender and cooked through. When you remove it from the heat, stir in the fresh thyme and let cool to room temperature.
2. Preheat the oven to 400F (200C)
3. In a large bowl, crumble the goat cheese or ricotta, then mix in the egg, crème fraîche or cream, milk, chorizo, and a bit more salt and pepper along with the onions.
(If you wish to use any other chopped, fresh herbs, add them now as well.)
4. Scrape the filling into the pre-baked tart shell and baking until just set and slightly-browned on top, 20-30 minutes. (As you can see, I used a rectangular mold. If using a different-sized tart pan, the baking time may vary. In which case, bake until it just feels set in the center.)
5. Let the tart cool briefly, then serve either warm or at room temperature. Sprinkle fresh thyme leaves over the top of the tart before serving.