Put the frozen spinach into the center of a clean kitchen towel and wring it over the sink to get all the water out.
Put the spinach in a medium bowl. Microwave the cream cheese about 20 seconds. Work it into the spinach with a fork.
Add the cheddar, onion, salt and pepper. Mix well.
Preheat the oven* to 400 degrees F. Line a large baking sheet with parchment paper.
Heat 3 tortillas at a time wrapped in a wet paper towel about 20 seconds in the microwave or until warm enough to work with so they don't break while rolling.
Place about ¼ cup of the filling on the end of a tortilla and spread it out like a log. Roll it up and place seam side down on the baking sheet.
Repeat until all the filling is used up. Brush them lightly all over with the melted butter.
Bake 14 - 18 minutes until lightly golden.
*Or pan fry in batches. Set a frying pan over medium to medium-high heat with enough oil to cover the bottom. Cook a few at a time, turning them to brown on all sides. Drain on paper towels.