Rinse and clean the spinach thoroughly.
Steam it in a large pot until it's wilted, which usually takes about 3 to 5 minutes.
Drain any excess water well.
In a skillet, melt the butter over medium heat.
Add the chopped onions and sauté until they become translucent, about 4 to 5 minutes.
Add the steamed spinach to the skillet with onions.
Stir in the salt and black pepper, mixing well to combine the flavors.
Reduce the heat to low and mix in the heavy cream.
Cook for about 2 minutes, allowing the cream to heat and blend with the spinach.
Sprinkle the shredded cheddar cheese on top.
Cover the skillet and let the cheese melt for about 2 minutes.
Serve warm.