In a large pot of salted boiling water, cook spaghetti according to package directions until al dente (tender yet firm). Drain and reserve 1 cup of pasta water.
While pasta cooks, make sauce: In a medium bowl, whisk eggs and Parmesan until combined. Set aside.
In a large skillet over medium heat, cook pancetta or bacon until crispy, about 3-4 minutes. Toss the minced garlic into the fat and saute for less than 1-minute to soften. Add drained tuna to skillet. Mix to combine. Alternatively, if omitting pork, add 2TBSP olive oil to skillet and follow the same instructions.
Add the hot, drained spaghetti to the pan and toss for about 2ish minutes to coat in the bacon fat with tuna*
Remove the pan from heat and pour the egg/Parm mixture into the pasta, whisking quickly until the eggs thicken, but do not scramble (this is done off the heat to ensure this doesn't happen).
Thin out the sauce with a bit of the reserved pasta water, until it reaches desired consistency (optional).
Serve Tuna Carbonara in bowls, and garnish with fresh black pepper, chopped parsley and additional Parmesan. Dig in!