For the ‘spanakopita’, heat olive oil in a large frying pan and sauté onion and garlic over low heat until soft and translucent (4-5 minutes). Add spinach and cook until just starting to wilt (3-4 minutes). Remove from heat, transfer to a colander and squeeze out excess moisture. Transfer to a bowl and cool, then add cheeses. Combine well and season with sea salt. Set aside.
Meanwhile, heat vegetable oil in a large saucepan. Add mussels, shallots, garlic and wine, then cover and cook over high heat, shaking pan occasionally, until mussels open (3-4 minutes). Drain mussels, reserving 100 ml cooking liquid and discarding any unopened mussels. Cool, then cover and refrigerate until cold (1 hour). Preheat oven to 180°C. Fill each mussel with a tablespoon of ‘spanakopita’ mixture, then close shells and tie firmly shut with kitchen string.
Heat reserved mussel stock, chicken stock, tomato and chickpeas in a saucepan over medium-high heat. Add mussels and bring to a simmer, then bake, covered with lid, until heated through (5-6 minutes). Serve immediately with white anchovies, if desired.