Roast the beets: Heat the oven to 400°F. Line a rimmed baking sheet with foil. Put the beets, herbs, salt, and a drizzle of olive oil in the center; toss the beets to coat. Fold the foil into a loose-fitting but tightly sealed packet around the beets. Roast the packet on the baking sheet until the beets are tender, about 1 hour and 20 min. Let the beets cool completely in the foil. When cool, use a paring knife to peel and slice the beets into wedges (six to eight per beet.) The beets can be roasted up to two days ahead and refrigerated.
Make the vinaigrette: In a small bowl, combine the mustard, vinegar, lemon juice, 1?4 teaspoon salt, and a few grinds of pepper. Slowly whisk in the oil.
Toast the pepitas: Heat oven to 375°F. In a small bowl, toss pepitas with corn oil, chile powder, and kosher salt. Spread evenly on a rimmed baking sheet and roast at 375°F until golden and fragrant, 6 to 8 min. (you'll hear them popping). Let cool completely on the baking sheet. If making ahead, store in an airtight container.
Make the salad: Combine the -cabbage and onion in a medium bowl and set aside. Up to an hour before serving, add the beet wedges to the cabbage and onions; toss gently with half of the vinaigrette.
Just before serving, add the baby greens, half of the feta, and half of the pepitas; toss with the remaining vinaigrette. Arrange on a big serving platter and garnish with the remaining feta and pepitas.