Peel the rutabagas, making sure to cut off all of the thick outer skin. Quarter length-wise and slice crosswise about 3/8-inch thick.
In a large soup pot, melt butter with thyme and bay leaf over medium heat until just starting to brown. Add leeks, cook for 3-4 minutes, then add rutabaga slices and potato chunks.
Stir in about 2 teaspoons salt, partially cover pot and cook until vegetables begin to soften, about 5 minutes.
Add stock, bring to a boil, lower heat and simmer 20-25 minutes, until vegetables are tender but not mushy.
Purée soup according to desired texture (purée half the vegetables with some liquid for a chunkier finished texture, or purée the whole pot for a smooth, creamy consistency). Season to taste with additional salt and pepper.
Stir the smoked paprika into the melted butter and top each bowl with a drizzle of the smoky butter.
Or top with croutons: chop day-old crusty bread into 1 cup of a bite-size chunks. Melt butter in a skillet and cook bread chunks over low heat until crisp and golden, then toss with the smoked paprika butter.