Whisk mayonnaise, sour cream, tarragon, lime zest and juice, salt and pepper; toss with cabbage and green onions and chill at least 15 minutes.
Cook the onion in oil over medium heat for 3 to 4 minutes.
Add garlic and spices and cook about 1 minute.
Add beans with their liquid and the water; bring to a gentle simmer.
Simmer partly covered for 15 to 20 minutes until thick and glossy.
Check that a spoon trail closes in about 2 seconds.
Stir in lime juice off the heat and adjust salt.
Char tortillas 20 to 30 seconds per side.
Fill with beans, slaw and avocado.