Season the fish with salt and pepper and place in a medium pot.
Cover with olive oil and add the onion, garlic, lemon, herbs, chili, whatever you like. The oil should come up almost over the top of the tuna (depending on the thickness of it, you may need more oil).
Turn the heat to low/medium-low and keep an eye on it, making sure the oil doesn’t get too hot too fast. Keep it at a gentle, steady, hot heat, without seeing more than the occasional bubble from the oil.
Cook the fish until it’s firm and has turned white and opaque, 12–20 minutes.
Remove from heat and let cool in the oil (this is the oil you can make an aioli with). You can eat the tuna warm, room temp or cold.
Transfer the tuna to a large serving platter or shallow bowl and spoon some of the cooking oil over. Transfer all but ¼ cup of oil to a glass jar or other container for later use.
Add Blue Runner Great Northerners to the oil in the pot and season with salt and pepper. Cook over medium heat until they’re warmed through and sizzling at the edges, 10–15 minutes.
To serve, toss celery and parsley together in a medium bowl and season with lemon, salt and pepper.
Spoon a little aioli or mayonnaise onto the bottom of a plate or bowl and top with the warmed beans.
Using a spoon or fork, break off large pieces of fish and place on top of the beans, followed by the parsley salad.
Combine the olive oil from the tuna and the canola oil in a measuring cup.
Whisk egg yolk and mustard in a small bowl and season with salt.
Whisking constantly, slowly drizzle in the oil, drop by drop at first, then a little more steadily, until aioli is starting to thicken. Eventually it’ll be thick like mayonnaise, that’s when you want to add a little squeeze of lemon to thin it out. This will allow you to continue adding the oil without the aioli breaking on you.
Once all the oil is added, season aioli with salt, cracked pepper and more lemon juice, if you’d like. It can also be thinned with more lemon or water if you prefer this as more of a saucy dressing.