Combine the soy sauce, rice vinegar, ginger, garlic, and sesame oil in a bowl. Add the sliced bison and toss to coat. Let marinate for at least 10 minutes.
Set up a steamer basket over a pot of gently simmering water. Arrange the bison slices in a single, even layer on a heatproof plate that fits inside the steamer.
Remove the bison from the marinade and steam for 4 to 5 minutes, until just barely cooked through with no pink remaining anywhere.
Whisk together the tahini, soy sauce, rice vinegar, and honey in a small bowl. Add water a tablespoon at a time until the dressing reaches a pourable, drizzling consistency.
Divide the rice or quinoa evenly among bowls. Top with the watercress, carrots, radishes, avocado, and steamed bison, adding a soft-boiled egg half to each if using.
Drizzle the dressing evenly over each bowl just before serving.