1. Rinse mussels under cold water and remove any beards; set aside.
2. Lightly toast California walnuts in oil.
3. Add coconut milk, tomatoes, garlic, thyme, and salt.
4. Cook, uncovered, until broth thickens, 3 to 4 minutes.
5. Toss in mussels, cover and cook until all have opened, 2 to 3 minutes.
6. Stir in arugula and remove from heat. Discard any mussels that have not opened.
7. Divide mussels and broth among service bowls. Sprinkle with feta cheese and serve with toasted or grilled bread.