In a large pot, melt the butter and cook the onions until tender. Stir in the carrots and jalapeno and toss to coat. Stir in the squash and potato. Pour in the chicken stock and bring to a simmer. Simmer until the vegetables are tender, about 30 minutes. Puree the soup in a blender in batches. Return to the pot and stir in the cream. Season to taste and garnish with the sliced scallion and bell pepper.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.