In a small bowl, whisk together 2 Tbsp rice vinegar, ¼ cup sesame oil, 1 Tbsp soy sauce, and lime juice.
Slice the bell peppers, zucchini, and carrots into long, thin strips. place in a resealable bag and pour in the marinade. Seal the bag, removing as much air as possible, and place in the fridge for at least 1 hour, or up to overnight.
In a small bowl, whisk brown sugar, miso, remaining rice vinegar, sesame oil, soy sauce, ginger, and black pepper to form a smooth sauce. Pour into a small saucepan over high heat and bring to a rolling boil. Boil for 1 minute stirring constantly. Remove from heat and set aside.
Over high heat, pre-heat a grill or cast-iron frying pan with a little heat tolerant oil. When the pan is very hot, add one tuna steak to the pan. Sear 30 seconds, until lightly golden, then flip the steak and brush with glaze. Remove steak from the pan and place on a serving plate, then brush each side with glaze. Repeat with remaining steaks. Once all steaks are cooked, brush with glaze again and tent with foil to keep warm.
Remove the vegetables from the marinade and add to the grill or frying pan. Sear for 7 to 10 minutes, tossing once or twice so they cook evenly. Place on serving plates with tuna and serve immediately, with additional glaze on the side.
The tuna is best enjoyed the day it's made. It does not store well. Leftover vegetables can be kept separately in an airtight container for up to one week.