In your medium saucepan, heat the olive oil over medium heat. Add the diced white onion and sauté for about 3-4 minutes until they become translucent. Then, add the seeded and diced jalapeño, cooking for an additional 2 minutes to soften.
Reduce the heat to low and stir in the half and half. Gradually add the shredded white American cheese and pepper jack cheese, stirring continuously until everything is melted and creamy.
Once the cheese is melted, fold in the drained diced tomatoes with green chiles. Then, add the chopped fresh spinach and stir until the spinach wilts and is well combined with the cheese mixture.
Remove the pan from heat and stir in the chopped fresh cilantro. This adds a wonderful freshness that elevates the flavor of the queso.
Transfer the White Spinach Queso to a serving bowl and serve immediately with tortilla chips or your choice of dippers.
