In a 5 quart dutch oven, heat oil over medium high heat until it ripples. Add shallots; saute for 1 minute then add broccoli/cauliflower mix. Season with celery salt, pepper & garlic powder. After 5 minutes remove broccoli mixture from pot and set aside.
In the same pot heat butter, when completely melted add flour. Stirring constantly with a whisk for 7 minutes or until flour smell is gone. Deglaze with sherry, (continuing to whisk) let alcohol burn off (1 minute) & start adding broth, slowly. When all broth has been added, season again adding Worcestershire sauce & bring to boil. Reduce to low & simmer for 10 minutes.
Add broccoli mixture back into the pot, as well as broccoli in cheese sauce package, sour cream and cheddar cheese. Using your whisk again, stir until all ingredients are aclumated
Season once more with Tabasco and nutmeg. Check for saltiness and if it needs more add sea salt and some extra black pepper to finish.