In a pot, saute onions 3-4 minutes in oil over medium high heat. Add garlic and sundried tomatoes and continue cooking until the mixture starts to caramelized.
Add wine and scrape the browned flavor from the bottom of the pan. Reduce the wine by half then add lemon juice, lemon zest and tomato juice.
Once that comes to a simmer, stir in the mussels, season with pinch of cayenne, salt and pepper, and cover the pot with a lid.
Cook 5-7 minutes or until they open. Sprinkle with freshly chopped parsley and drizzle with extra virgin olive oil.
Arrange spinach in a large bowl. Pour mussels and sauce over the spinach and toss with tongs until the spinach wilts. Serve immediately, discard any mussels that don’t open.