Trim just enough off the root end of each onion so it sits flat, then cut about ¼ inch off the top (stem end). Place the onion on a cutting board, root side down. Starting at the top, make 3 evenly spaced cuts down through the onion, as if cutting a pie into 6 wedges. Cut only about two-thirds of the way down, leaving the bottom third intact so the onion stays together and opens like a flower. Peel off the papery outer skins.
Bring a large pot of salted water to a boil. Preheat the oven to 425 degrees and line a baking sheet with parchment paper.
Place the onions in the boiling water and cook for 4-5 minutes. Remove from the water and let cool until cool enough to handle. When cool enough, gently open up the layers of the onions so they look like flower petals. They will open up more while roasting so don’t force them. Drizzle each onion with a little olive oil. Place on the prepared baking sheet and roast for 40-45 minutes or until the onions are tender and the tips are starting to brown.
Combine the vinegar, honey, tamari, and sesame oil in a small pot or skillet and bring to a boil. Let boil gently for 2 minutes or so or until the sauce reduces and thickens almost to a syrup consistency.
Drizzle the onions with half the sauce and put back in the oven for 5 minutes. Put the onions on a serving platter, drizzle with the rest of the sauce and sprinkle with the furikake or everything bagel seasoning.