Heat oil in a saucepan over medium-high heat. Saute onion in hot oil until softened, 5 to 7 minutes. Stir tomatoes and jalapeno pepper into the onion; add spinach and garlic and cook until the spinach wilts, 3 to 5 minutes.
Pour half-and-half into the saucepan; add pepperjack cheese and cream cheese. Bring to a simmer, reduce heat to medium-low, and cook until the cheese is completely melted, 5 to 7 minutes.