Preheat oven to 350° and lightly grease cast iron skillet or a baking dish.
Dice onions, tomatoes, and jalapeno. Choose older jalapenos for more spice and add jalapeno seeds. Choose young jalapenos for less spice and discard the seeds. (I highly recommend wearing gloves when working with a jalapeno, that way your hands won't burn for hours.)
Combine all ingredients in a bowl and mix to thoroughly combine.
Spread the dip mixture in a cast iron skillet or a baking dish.
Top off with some more grated Monterrey Jack cheese, jalapenos, and diced tomatoes.