Bring all syrup ingredients to a boil in a 1-quart heavy saucepan, stirring until sugar is dissolved. Reduce heat and simmer until syrup is reduced to about 1/2 cup, about 15 minutes. Remove from heat and let steep, covered, 1 hour. Pour syrup through a sieve into a bowl, discarding solids. Heat a dry heavy 10-inch skillet over moderate heat until hot, then toast pumpkin seeds, stirring constantly, until puffed and beginning to pop, 3 to 5 minutes. Transfer seeds to a plate and season with salt. Halve, pit, and peel avocados, then thinly slice lengthwise. Gently toss slices with 2 tablespoons lemon juice and salt and pepper to taste, then gently toss with 1/4 cup syrup. Halve pears and core, then thinly slice lengthwise. Toss pear slices with remaining 2 tablespoons lemon juice and salt and pepper to taste. Stir in remaining 1/4 cup syrup. Drain pears, reserving syrup, and arrange slices on 4 plates. Transfer avocado slices to plates with a slotted spoon, discarding syrup. Toss greens with reserved syrup from pears and salt and pepper to taste. Mound greens next to avocado and pear slices and sprinkle with pumpkin seeds.