Use a spiralizer to create noodles from the zucchini, then place them on a paper towel and sprinkle with a pinch of sea salt to draw out excess moisture.
In a small dry skillet over medium heat, toast the pine nuts for 2-3 minutes until they are golden and fragrant, then set them aside.
Pat the tuna steak dry with a paper towel and season both sides evenly with the sea salt and black pepper.
Heat half of the olive oil in a cast-iron skillet over medium-high heat and sear the tuna for 2 minutes per side for a medium-rare finish.
Remove the tuna from the pan to rest on a cutting board, then add the remaining olive oil and minced garlic to the same skillet, sautéing for 30 seconds.
Add the zucchini noodles to the skillet and toss for 1-2 minutes until they are just tender but still retain a slight bite.
Stir in the lemon juice, lemon zest, and chopped fresh parsley, then immediately remove the skillet from the heat.
Slice the tuna steak into thin strips and serve it atop the zesty zucchini noodles, finishing the dish with the toasted pine nuts.