Get a flat pan and cook them on your stovetop, not the oven. Put the burner on medium to medium/low. Butter the bread and cook the sandwiches until the bread toasts. By that point the cheese should be melted. If it toasts too fast you can easily control the heat.

Here’s a link to some grilled cheese and tomato basil soup that I make for the wife and kids. The bread is sourdough and I used cheddar and Monterey Jack cheese.