Preheat the oven to 350 degrees Fahrenheit. Mix the tuna, 1/4 cup of the olive oil, onions, salt, pepper, fresh thyme, and Dijon mustard together in a large bowl. Cover and refrigerate for 1 hour. Meanwhile, cut the top 1/4 off the garlic clove, place it on a piece of tin foil, drizzle it with olive oil, wrap it in the foil, place it on a baking sheet and roast it in the oven for 1 hour. Then remove it from the oven and allow to cool before squeezing the garlic pulp out of the clove into a small bowl. Mix the garlic with the mayonnaise, then cover and refrigerate it until use.