Over low heat, melt butter in a heavy 5-quart pot and heat until foaming.

Add carrots and onions, cover, and cook until soft but not brown, 15-20 minutes.

Stir in garlic and stock and bring to the boil over medium heat.

Reduce heat, cover, and simmer until vegetables are very soft, about 40-45 minutes.

Remove solids and puree in batches (adding cooking liquid as needed) until very smooth.

As each batch is pureed, force through a fine-mesh sieve.

Strain cooking liquid and stir in puree.

(At this point the mixture may be refrigerated for several days.)

Bring mixture just to the boil, add cream, and warm through (but do not return to the boil).

Season with salt and pepper.

Garnish with snipped chives and serve.