Step 1Grind the English muffin to crumbs in a food processor. If fresh, dry the crumbs in a microwave oven or by tossing in a skillet.
Step 2Meanwhile, puree the onion in the blender. Strain the onion juice through a strainer and discard the solids. Mix the onion juice with the crumbs and squeeze out any excess juice.
Step 3Place the crumbs, beef, salt, pepper and a few grindings of nutmeg in a food processor and process until it forms a ball on top of the blade, or knead thoroughly by hand. Form the meat into 4 patties and dust on both sides with flour.
Step 4Heat about 2 tablespoons of oil in a large skillet over medium-high heat until a corner of one of the patties sizzles immediately if dipped into the oil. Fry the patties until browned on both sides, 3 to 4 minutes, and remove to a warm platter.
Step 5Pour off the oil. Add the chicken broth and scrape up the browned bits from the skillet. Bring to a simmer. Stir the flour into the sour cream until smooth. Add the sour cream to the skillet and return to a simmer, stirring, 1 minute. Serve the patties with the sour cream sauce.