Just can't master the RECALL. Help!

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The facts are these:
- suspected Beagle/Coonhound mix
- ~6yrs old, adopted from kennel
- we've had her for a little over 8 months
- learned excellent recall on leash, indoors
- learned excellent 'sit', 'wait', 'down' commands

Problem: the Recall outdoors, off-leash.
Once she gets her nose to the ground, it's like nothing else exists! We've tested her a few times at the cottage and she will wander off (and eventually get lost).
I've tried:
- positive training, repeat successful recalls all the time indoors and on-leash outside
- tried (and currently training when I feed her) with a dog whistle...
- all kinds of treats: dog treats, bacon, goldfish crackers, hotdogs, raw meat.

Nothing is as enticing as all those SMELLS!
Hound-owners, please tell me there's a good training trick for this!