Heat olive oil and butter together in a 3-quart saucepan over medium heat; cook and stir onion until translucent, about 10 minutes. Add garlic and cook until fragrant, about 1 minute. Stir red bell pepper and jalapeno pepper into onion mixture; cook until softened, about 5 minutes.

Reduce heat to low and mix pepperjack cheese and spinach into onion mixture; cook, stirring constantly, until cheese is melted and mixture is thick, 5 to 10 minutes. Slowly pour enough evaporated milk into cheese mixture, stirring constantly, until desired consistency is reached; season with salt. Continue to cook and stir over low heat until warmed through, at least 5 minutes more.