Combine the drained tuna, Sriracha, Greek yogurt, mustard, and onion (optional) in a food processor. Process until smooth, then transfer to a small bowl.
Add lemon zest and season with salt and pepper to taste. Mix well.
Chill the tuna spread in the refrigerator for at least 15 minutes to let the flavors meld.
Serve on your favorite bread, tortilla, bagel, crackers, or lettuce wraps. For extra texture and flavor, top with sliced tomatoes, cucumbers, pickles, or hard-boiled eggs.
Store tuna spread in an airtight container in the refrigerator. It will stay fresh for about 3 days.
When making and traveling with a sandwich: Wrap the sandwich in parchment paper or foil to keep it together. It’s important to keep it chilled with an ice pack, especially for longer days. Lettuce, tomato, or pickles can be added just before eating. If kept at room temperature, eat it within 2 hours.If stored with an ice pack, it can last up to 4-5 hours safely.