In a medium saucepan, melt butter over medium heat. Add onion, jalapeno, and salt. Cook until onions are softened, about 3 or 4 minutes.
Sprinkle flour over mixture and cook for about one minutes, until thick and evenly coated. Slowly add the milk and cook, stirring until bubbly.
Remove from heat and stir in cheese until smooth and melted. Add in spinach and tomatoes and serve warm! We like ours with chips and/or pretzel buns.
Notes
Like all queso recipes, this dip is best served warm. Go ahead and dig in right away!
The queso will get firmer as it cools. If this happens, just reheat in the microwave, stir and serve. You can also keep it warm in your slow cooker on low and enjoy it throughout the day.