1. Combine vinegar, 1/3 cup maple syrup, 2 tablespoons mustard, and thyme in a shallow dish or large zip-top plastic freezer bag; add steaks, turning to coat. Cover or seal, and chill 2 hours, turning occasionally.
2. Heat olive oil in a large heavy skillet over medium-high heat; add pecans, stirring to coat. Stir in 2 tablespoons maple syrup, and cook, stirring constantly, 1 minute. Remove pecans from pan, and spread on wax paper to cool.
3. Remove steaks from marinade, reserving marinade. Sprinkle steaks evenly with salt and pepper.
4. Grill, covered with grill lid, over medium-high heat (350° to 400°) 4 to 5 minutes on each side or to desired degree of doneness. Keep warm.
5. Whisk together reserved marinade and remaining 2 tablespoons mustard in a small saucepan. Bring to a boil, and cook at least 2 minutes. (Sauce should be thick and syrupy.)
6. Cut steaks into 1/2-inch-thick slices; sprinkle with pecans and blue cheese. Serve with sauce. Garnish, if desired.