Butter the bread on one side of each slice. Spread mayo on the bread, opposite side of the butter and spread the boursin cheese on the other slice of bread.
Spread out the avocado on top of the boursin, add the slice of havarti on top. Then add the other slice of bread, mayo side to the havarti cheese. The butter should be on the outside of the sandwich.
Heat a pan over medium low heat. Add the sandwich to the pan and cook for 2-3 minutes per side, or until golden and crispy. Turn down the heat if your sandwich is getting too dark. If the cheese hasn't fully melted, turn the pan to low and cook until cheese is melted.
Notes
Any type of Boursin will work, but I used herb and garlic.