This is a superior version of what Westerners usually refer to as tandoori chicken (which is silly, because anything cooked in a tandoor — a very hot clay oven — is necessarily tandoori), properly called chakundari chicken tikka.
For the first marinade, combine the lemon juice, 1 teaspoon each of the garlic and ginger and salt in a large bowl. Rub the mixture into the chicken thighs, and marinate for at least 20 minutes at room temperature — or longer in the refrigerator, covered.
Put the beets and yogurt in a food processor and purée until smooth. Add the remaining salt, garlic, ginger, dried fenugreek leaves, garam masala, cumin and canola oil and pulse to combine.
Coat the chicken with the puréed beet mixture and marinate, refrigerated and covered, for a least 1 hour (or up to 8).
If you’re using wooden skewers, soak them in water first. Heat a charcoal or gas grill or a broiler to medium-high, and put the rack at least 4 inches from the heat source. Skewer the chicken, drizzle with melted butter and set on the grill or broiling pan. Cook, rotating carefully to cook and brown on all sides, about 8 minutes total. Garnish with lemon wedges.