Combine mangos, mustard, sugar and thyme in a medium bowl.
Season with salt and pepper to taste, set aside.
Preheat outdoor grill or indoor grill pan over medium-high heat.
Place four bratwurst on hot grill, cover and cook for 15 to 20 minutes; turning often until internal temperature reaches 160 degrees.
To serve, place cooked bratwurst in hot dog bun and top with mango relish.
Makes about 1 cup relish (enough for four bratwurst).