- Thinly slice onions and set aside.
- In a medium frying pan, heat up oil and add onions, cook on a low flame for 10 minutes until softened and golden. Season with thyme, salt and pepper to taste. Remove pan from heat, leave to cool. Preheat an oven to 425 F.
- To a large bowl, sift flour, baking soda and salt to make the base. Use pastry blender or two knives to rub the butter into the dry ingredients until the mixture looks like coarse breadcrumbs. Add milk, Stir with a wooden spoon until a soft dough forms. Turn the dough onto a lightly floured countertop and knead lightly.
- Pat dough out into 8 inches circle, transfer to baking tin. Press the dough down into a thin, even layer and cover with onions.
- In a small bowl, add egg and cream, beat them together. Pour over the onions, garnish with poppy seeds and nutmeg. Bake now for 30 minutes until egg is puffed and turns golden.
- Leave to cool the tart for 10 minutes. Slip a small sharp knife around the edge between the pastry and the tin to loosen slightly then carefully unmold onto a plate. Make tart wedges and serve really hot to all.