Bring a large pot of lightly salted water to a boil. Add linguine pasta, cook for 8 to 10 minutes, until al dente, and drain.
Heat 2 tablespoons olive oil in a large saucepan over medium-high heat, and cook the onion 4 minutes, until tender. Mix in the thyme, and continue cooking 2 minutes, until onion is golden brown. Set aside onion and thyme. Place the tuna in the saucepan, and cook 2 to 3 minutes, until browned. Set aside, and keep warm.
Heat the remaining oil in the saucepan. Return the onion and thyme to saucepan, and stir in the white wine. Cook until reduced by about 1 tablespoon. Mix in the chicken stock, lemon juice, and lemon zest. Reduce heat to medium, and continue cooking 10 minutes, until reduced to about 3/4 cup.
Mix the sun-dried tomatoes and artichoke hearts into the saucepan, and cook just until heated through. Return tuna to the saucepan, and cook to desired doneness. Toss the cooked pasta into the saucepan. Season with salt and pepper.