Heat the olive oil in a medium sized sauce pan over medium-high heat.
Add the shallot and sauté for a couple of minutes.
Add the garlic, season with salt and pepper, and cook for another minute. Make sure to continuously stir so you don't burn the garlic.
Add the chardonnay and reduce the sauce until it's a sauce like thickness (approx 3-5 minutes, but cook it down more if you prefer a thicker sauce),
Once the sauce is thicker, remove the pan from the heat, and add the butter, a little at a time. Keep adding butter until you get a taste you like. I used 1 stick and 1 tablespoon of butter. My buddy uses 2 sticks of butter.
Once you like the taste, slowly start to infuse the lemon juice. Continuously whisk the sauce as you slowly add the lemon juice. If you stop whisking, the sauce can break. While whisking, have a spoon in reaching distance so you can taste the sauce. Stop adding the lemon juice when you're happy with the taste. I did not use the juice from 2 lemons, but I used more than one.
Now add the parsley and capers, stir.
Taste for salt and pepper.
While the sauce was reducing in the wine stage, I heated olive oil over medium-high heat. Salt and pepper both sides of the tuna steaks. When the pan is hot an the olive oil is slightly smoking, add the tuna steaks and sear for approx 2-3 minutes per side. You want to ensure the middle is still rare, but the edges have a nice brown color.
When the tuna steak has finished cooking, serve it over cabbage (optional) and top with the Beurre blanc sauce.