Melt butter in a frying pan and saute red onion until soft and translucent. Once done, keep it aside and cool down.
Preheat the oven to 325F temperature and line muffin tray with muffin liners.
In a small mixing bowl, add milk and vinegar. Mix well and keep it aside.
In a large mixing bowl, add all purpose flour, cheddar cheese, red chilli powder, baking powder, baking soda and salt. Mix well.
Add the milk-vinegar mixture to this and mix using a spatula. It may look dry initially but blends well when you keep mixing it.
Add sauteed onion along with melted butter. Then add finely/roughly chopped fresh spinach leaves. Mix well.
You get a thick mixture and make sure there are no dry spots. If the mixture is very dry, add 1 or 2 tbsp milk at a time just to bind the mixture uniformly.
Scoop the dough to the muffins liners uniformly.
Bake it for 30 to 35 minutes or until the toothpick or skewers inserted into it comes out clean and the muffins are golden brown in colour.
Yummy Eggless spinach and cheese muffins are ready. Serve warm.