This dip tastes best cold. Chill the avocados in advance if serving the dip immediately. Baby spinach can be used, but regular spinach will provide more texture and fiber.
Heat oil in large nonstick skillet over medium heat. Cook shallots and garlic, stirring often, until tender, about 4 minutes. Add spinach and 3/4 teaspoon salt. Cook, covered, until wilted, about 4 minutes. Uncover. Cook until liquid evaporates, about 7 minutes. Refrigerate until cooled, about 25 minutes.
Meanwhile, puree avocados, yogurt, lemon juice, and 1/4 teaspoon salt in a food processor until smooth. Season with pepper. Finely chop spinach mixture. Stir in avocado puree. Serve with snap peas and bread for dipping.
Dip can be refrigerated with plastic wrap pressed directly onto surface for up to 2 hours.