Combine tomatoes and cumin in a medium saucepan and bring to a simmer over medium-low heat. Add cheese a handful at a time, stirring, just until melted, about 2 minutes.
Stir in jalapeños and milk. Simmer over low heat, stirring constantly, until mixture is well combined, 2 to 4 minutes. Don't overheat; cheese will seize and become clumpy.
Stir in cilantro; season with salt and pepper. Keep dip warm in a slow cooker set on low or a pan over very low heat, stirring occasionally. Serve with tortilla chips, if desired.
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