Saute onions. Heat oil in a large stock pot over medium heat. Add onion and season with salt and pepper. Cook, stirring occasionally for 3-4 minutes.

Cook remaining vegetables. Add jalapeños, garlic, and potatoes and season again with salt and pepper. Cook until vegetables are starting to soften, about 4 minutes more.

Bring to a simmer. Add tomatoes, milk, and stock and bring to a simmer, stirring occasionally. Cover and simmer over low heat until potatoes are tender, about 20 minutes.

Add cheese. Taste and season with salt as needed. Add cheese and stir to combine. Let sit a minute or two then ladle into bowls and serve with tortilla chips.