In a large microwave-safe bowl, stir together semisweet chocolate, butterscotch chips, and butter. Cook in 20-second increments, stirring after every 20 seconds.
Let cool slightly and then add marshmallows and walnuts. If you add the marshmallows while the chocolate is still hot, the marshmallows will melt.
Line baking sheet with parchment or waxed paper. Scoop 2-Tablespoons mounds of the chocolate rocky road mixture onto lined baking sheet. Sprinkle with flaky sea salt.
Refrigerate until clusters are firm and chilled, about 30 minutes.