Beet Aguachile

Preheat oven to 400°F. Place beets in a rimmed baking dish; cover with kosher salt. Roast in preheated oven until beets are tender when poked with a skewer, 50 minutes to 1 hour. Remove from oven; let cool slightly, about 20 minutes. Uncover beets; using a paring knife or a paper towel, remove and discard skin. Cover beets; refrigerate until ready to use or up to 2 days.

While beets cook, combine oil, beet juice, lime juice, 1 cilantro bunch, shallot, vinegar, orange zest and juice, serrano chile, garlic, crushed chiltepin chiles, black garlic (if desired), and fine sea salt in a blender. Process until smooth, about 1 minute. Let stand, covered, in refrigerator 1 hour to allow flavors to meld. Pour through a fine wire-mesh strainer into a bowl, pressing on solids to extract all liquid; discard solids. Cover and refrigerate until completely chilled, at least 1 hour or up to 12 hours.

Remove beets from refrigerator. Cut lengthwise into quarters or sixths. Remove leaves from remaining cilantro bunch; discard stems. Ladle chilled beet broth onto a large plate, or ladle evenly into 4 shallow bowls. Arrange beet slices evenly among bowls. Top evenly with radishes, cucumbers, onion, and cilantro leaves. Serve with tostada shells.