Is it permitted to eat tuna with milk?

Shalom,
 
The custom of Sephardim is to refrain from eating fish with milk - therefore they should not eat a pastry that contains both milk and tuna. However, the custom of Ashkenazim is to be lenient and eat fish with milk. Indeed, those who have accustomed themselves in their family to refrain from this should continue with their practice.

Sources: The custom of Sephardim to avoid eating fish with milk is based on the words of the Beit Yosef in Yoreh De'ah (Siman 87), who wrote that one should not eat fish with milk due to the danger it poses. And see extensively on this matter in the responsa Yechave Da'at Part 6 (Siman 48), where it was established that Sephardim should refrain from eating fish with milk or cheese due to safety concerns. Yet those who are accustomed to eat fish with butter may continue their practice. And see also in responsa Yabia Omer Part 10 (pages 303-304) and in responsa Shemesh u'Magen Part 4 (Yoreh De'ah Siman 12), and in responsa Divrei Eliyahu Part 9 (Siman 50) and in responsa Berachat Yehuda Part 2 (Yoreh De'ah Siman 5) and in responsa Ya'alet Chen Part 1 (Yoreh De'ah Siman 2).

However, the custom of Ashkenazim is to be lenient regarding eating fish with milk, see the Rema in Darkei Moshe (there, paragraph 4) who cited the aforementioned words of the Beit Yosef and wrote, "I have not seen any who are careful about this." The same is written by the Shach (section 6) that we have not heard or seen anyone refrain from eating fish with milk, and every day they are accustomed to cooking fish with milk and eating it. And this is also the opinion of the Taz (section 3) and the Magen Avraham (beginning of Siman 178) permitting the eating of fish with milk. And so it is written in the mentioned Yechave Da'at that those of Ashkenaz are accustomed to be lenient in all this, and they have what to rely on.

Indeed, there are among Ashkenazim those who have accustomed themselves in their family to refrain from eating fish with milk, and they should continue their practice, see in the book Hilkhot Shlomo on the festivals (Nisan–Av, chapter 12, note 48) and in responsa Even Penei Part 1 (Siman 121). And see also in the book Sha'arei Halacha u'Minhag Part the addenda and supplements (Siman 99) regarding the custom of Chabad on this matter.

Blessings,
 
Hillel Meirs