In a medium bowl, combine all 3 cheeses and the basil. "Grating the harder cheeses makes the pieces all the same size so they melt evenly. And adding a creamy cheese, like fromage blanc or cream cheese, distributes the grated cheese evenly over the bread," Conley says. All of this makes for a very creamy filling.
Top 2 bread slices with cheese mixture, smushing it evenly onto the bread. "Stale bread works really well for grilled cheese," says Conley. "It's a little drier, so it browns better." Form sandwiches with remaining 2 slices. It's a generous amount of filling, but that's okay. However, if you're using a fluffier, lighter bread, she advises using a little less, so the bread isn't overwhelmed.
Butter the outsides of each sandwich--"all the way to the edges, so every part gets toasted," says Smith. The Cowgirls like Straus Family Creamery butter from nearby Tomales Bay, but any butter is fine.
Heat a large well-seasoned cast-iron skillet or a large, heavy nonstick pan over medium heat for a minute. The heavy bottom is important: It conducts heat evenly, so your sandwiches won't scorch.
Place 2 sandwiches in skillet and cook until golden-brown underneath, 5 to 7 minutes. Flip and cook the other side. "The most common mistake in making grilled cheese is cooking it too hot," says Conley. "Low and slow is the key." Cheese will probably ooze out, but that's okay; it gives the sandwiches a nice crunchy fringe.