In a large stockpot, saute onion in butter until just tender.
Add potatoes, chicken broth, and cumin, then cover and cook until potatoes are tender (about 20 or 30 minutes).
Remove from heat and add jalapenos, with liquid, baking soda (to prevent curdling), and evaporated milk.
Coarsely mash potatoes in the pot with a potato masher.
Stir well, then season with salt and pepper; return to heat and simmer for 15 minutes, stirring frequently.
Garnish individual servings with a dollop of sour cream and a sprinkling of chopped green onions.
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