In large pot of boiling water, blanch spinach, in batches, for 30 seconds. Using slotted spoon, transfer to bowl of ice water to chill. Drain well; squeeze out excess water. Coarsely chop; set aside. (Make-ahead: Refrigerate in airtight container for up to 24 hours.)
In large skillet, heat oil over medium-high heat; cook garlic until golden and fragrant, about 1 minute. Stir in mushrooms; cook, stirring often, until tender and light golden, about 7 minutes.
Stir in spinach, lemon juice, salt and pepper; cook, stirring, for 1 minute.
