Place spinach and 2 tablespoons water in a microwave-safe large bowl. Cover tightly with plastic wrap; cut a small slit in the center with the tip of a paring knife to vent excess steam. Microwave on high (at 100 percent power), stirring halfway through, until the spinach is just wilted, 3 minutes in an 1,100-watt oven or 6 minutes in a 700-watt oven. (When removing the plastic wrap, be careful to avoid the hot steam.)
Scatter the onions, garlic, butter, 1/4 teaspoon salt and a couple grinds of black pepper in a microwave-safe 8-inch square baking dish. Cover tightly with plastic wrap; cut a small slit in the center. Microwave on high (at 100 percent power) until the onions are soft and translucent, 5 minutes in an 1,100-watt oven or 8 minutes in a 700-watt oven. If the onions are still a little hard, microwave again, covered, in 30-second increments, stirring after each.
Squeeze the wilted spinach completely dry, and place in the baking dish with the onion mixture. Add the heavy cream, cream cheese, Parmesan, cayenne, nutmeg, 1/4 teaspoon salt and a few grinds of black pepper, and stir to combine. Microwave, uncovered, on high (at 100 percent power) for 5 minutes in an 1,100-watt oven or 7 minutes in a 700-watt oven. The spinach should be hot and bubbly and the cream slightly thickened. Add the lemon juice, and stir. Top with breadcrumbs or fried onions and some lemon zest.
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