In a medium saucepan over medium heat, melt 4Tbsp butter.
Sprinkle in 4Tbsp flour and whisk constantly for 1–2 minutes to cook the roux; do not let it brown.
Slowly pour in 3c. milk in a steady stream while whisking to prevent lumps. Continue whisking and bring the mixture to a gentle simmer until it thickens, about 3–5 minutes.
Reduce heat to low. Add 2c. Velveeta white queso shredded and 2c. Velveeta pepper jack shredded in batches, stirring after each addition until the cheese is fully melted and the sauce is smooth.
Stir in 1/2c. red peppers diced, 2Tbsp jalapenos diced, and 4oz frozen spinach thawed & drained (squeeze out excess liquid before adding). Stir just until the vegetables and spinach are evenly heated through; do not boil.
The recipe calls for 2Tbsp jalapenos, which will make the dip fairly spicy—if you prefer milder heat, add less than the listed 2Tbsp.
Transfer the dip to a serving bowl and serve warm with chips or bread. Keep warm over low heat or in a small slow cooker if desired.