Marinate fish before leaving home: In a small pot, combine tamari, birch syrup, chile, ginger, smashed garlic, 2 sprigs thyme, the sake kasu or miso, and 2/3 cup water (1/3 cup if using miso). Bring to a simmer and cook, whisking occasionally, 5 minutes (or, for miso, just bring to a simmer, then remove from heat without further cooking). Let cool. Pour into a 1-gal. resealable plastic bag. Add fish, seal, and freeze until solid, at least 6 hours. Pack in a cooler for the trip.
Thaw bagged fish in a bowl of cold water a few hours before cooking (or let thaw in cooler overnight). Remove from marinade and pat dry. Heat a charcoal grill to medium-high (450°) or set up a camp stove and use medium-high heat.
Heat 2 tbsp. butter in a large heavy frying pan or 2 smaller pans. Add fish and sauté, turning once, until crisp-edged and opaque in center, 10 to 15 minutes. Set pan aside, covered and wrapped in a towel to keep warm. If using charcoal, add 10 to 12 briquets to fire.
Melt 2 tbsp. butter in a medium saucepan, add onion and thyme leaves, and cook, stirring often, until onion starts to soften, 3 to 5 minutes. Stir in minced garlic, 1/4 cup water, the peas, and chard ribs and stems and cook, covered and stirring often, until tender, about 5 minutes. Stir in chard leaves; cook, covered, until wilted, about 2 minutes more. Stir remaining 1 tbsp. butter into vegetables and serve with fish.
*Order birch syrup through Kahiltna Birchworks (alaskabirchsyrup.com). Find sake kasu at Japanese markets.