Place all cheeses and onion into a food processor and pulse several times to mix. The finer the mixture, the cleaner and smoother it will melt.
In a medium sauce pan, melt butter and then add the cheese mixture through whole milk over medium heat. Continuously whisk the mixture until it is fully melted and smooth (maybe a few small bumps from the onions). Do not heat too high or keep it over heat.
Whisk in salt and pepper.
While still hot, transfer cheese to a serving bowl, top with diced chorizo (some will sink). Garnish with jalapeno. You can stir in the sausage to combine. Serve with tortilla chips.