Heat oven to 350 degrees F (177 degrees C).
Stir sour cream, mustard powder and thyme in a large bowl then add mozzarella, Monterey Jack and the cup of chopped basil leaves. Stir then add to a small baking dish or pie dish.
Bake for 10 to 15 minutes or until cheese has melted and is bubbling.
While the dip is in the oven, add bread slices, in one layer, to a baking sheet. Slide into the oven and bake 6 to 8 minutes until toasted.
After baking the cheese dip, turn oven to broil and broil the cheese dip 2 to 4 minutes until golden brown on top.
Serve warm with tomato and extra basil leaves scattered on top and alongside toasted bread slices.