Warm 1 tablespoon of the olive oil in a large skillet over medium heat.
Add in the onion, oregano and chili flakes, plus some salt and pepper, and sautée about 2 minutes, until softened.
Add the tomatoes and a little more salt and sautée for an additional 2 minutes.
Add the zucchini noodles and toss for a minute, until combined and warm.
Remove from heat, add in the remaining 2 tablespoons of olive oil, and a healthy sprinkle of salt and pepper. Toss to combine.
Top with the sardines or tuna.
Garnish with fresh oregano and lemon juice. Feel free to drizzle a little more olive oil if necessary.