Slice onions thinly on a mandolin.
Melt butter + oil in large pot over med-low. Add onions + salt, tossing to coat. Slowly cook until carmelized, 30-40 min. Add pepper.
Turn up heat to med-high, adding red wine. Cook until reduced, 2 min.
Add water, sherry, bouillon, rosemary + thyme. Bring soup to a simmer, cook 2 min. Remove from heat + remove sprigs.
Place rack in top third of oven + turn broiler on.
On a baking sheet, place oven-safe bowls + add soup to each. Leave space at top.
Spread cashew cream cheese on toasted slices + place two on each bowl. Bake with door slightly open until cheese is golden brown, 2-3 min.