Slice loaf of bread in half horizontally and place cuts sides up on a cookie sheet. Set aside.
In a large mixing bowl combine Monterey Jack cheese, Cheddar cheese, Parmesan cheese, garlic powder, smoked paprika, Ranch seasoning, sliced green onions, melted butter and mayonnaise.
Stir all the ingredients together until it comes together into a thick paste consistency. Add more mayonnaise, a tablespoon at a time if mixture looks dry, to obtain the desired consistency.
Spread a generous layer of the cheese mixture onto each side of the French bread loaf.
Bake for 12 to 14 minutes or until cheese is hot and bubbly.
Turn oven temperature to "broil" and toast until edges are toasted lightly golden brown.
Remove from oven and allow bread to sit for a few minutes before slicing each half into 2 inch thick slices.
Notes
Store any extra cheese spread in an airtight container in the fridge for up to a week.