Place the potatoes in a large pot and cover with cold water. Season generously with salt. Bring the pot of water to a boil over high heat. Once boiling, reduce to medium-high and cook the potatoes until fork tender, about 10-12 minutes.
While the potatoes cook, place the eggs in a medium pot and cover with 1 inch of cold water. Bring the water to a rapid boil over high heat. Once boiling, turn the heat off, cover the pot (but don’t remove it from the heat) and let the eggs stand for 10-12 minutes. Then, drain out the water and cover with cold water to chill until ready to use.
Bring another medium pot of generously salted water to a boil. Once boiling, add in the red pepper and asparagus, and cook just until fork tender, about 3 minutes. Immediately drain and transfer to a bowl of ice water to stop the cooking process.
Heat the 2 tsp of olive oil in a large pan over medium-high heat. While the oil heats, spread the sesame seeds into a shallow plate that has sides.
Lightly press each side of the tuna steaks into the sesame seeds until all sides are evenly coated.
Place the tuna into the hot pan and sear just until the seeds darken, about 1-2 minute per side, depending on how cooked you like your tuna. Make sure you sear all the sides, not just the front and back! Set aside while you make the vinaigrette.
Whisk all the vinaigrette ingredients, up to the olive oil, together in a medium bowl. Then, while constantly whisking, stream in the olive oil until well mixed.
Place the spinach, tomatoes, asparagus and bell pepper into a large bowl and toss with the vinaigrette. Divide between two large bowls.
Then, divide the capers, olives, eggs and potatoes (slice them if you wish) on top. Finally, slice the tuna and divide between the bowls.