Remove the leaves from the swiss chard stems and chop the stems into a similar size as the diced onions. Heat some olive oil in a medium pot and add the diced onion and chopped stems. Season with salt and pepper and cook on medium heat, stirring occasionally until onions are translucent and beginning to stick to the bottom of the pot. Add the chopped garlic, wait a minute and then add the sherry to deglaze.
Add the butter to the onion mixture and when it is melted add the flour. Stir until incorporated then pour in the milk. Add a pinch of nutmeg and heat to a simmer, stirring frequently.
Roll up the swiss chard leaves like a cigar and slice them into half inch ribbons. Add to the simmering milk and keep stirring until they are all wilted. Continue to cook on med - med low until greens reach desired tenderness. I cooked mine about 10 minutes. Add the cheese and then try not to eat it all straight out of the pot.