Preheat the oven to 400°F (204°C). Line a quarter sheet pan with parchment paper.
To prepare the bok choy, whisk together the sunflower oil and tamari in a small mixing bowl. Place the bok choy on the sheet pan, cut side up. Drizzle the oil and tamari mixture over the top of the bok choy as evenly as you can.
Place the bok choy in the oven and roast for 10 minutes. Take them out, turn them over so they are now cut side down and bake for another 5 to 10 minutes, until the leaves are charred.
At the same time, prepare the tempeh. Add the tempeh to a pan with sides and cover with water. Bring to a boil, reduce to a simmer and cook, uncovered, for 10 minutes.
Add the miso, water, mustard, tamari and sunflower oil to a big mixing bowl. Break up the miso with a small silicone spatula and then whisk the marinade together until completely smooth.
When the tempeh has finished simmering, drain it and drop it into the marinade, flipping it over to coat both sides. Marinate for 10 minutes.
Heat a cast-iron or nonstick skillet over medium heat. Add the tempeh and cook until the first side starts to brown and the marinade dries up a bit, about 5 minutes. Flip the tempeh over and cook the second side, until it starts to brown, another 5 minutes.
Serve with a drizzle of lemon juice and a sprinkle of sesame seeds.