Prep the spinach. Drain the thawed spinach in a colander, then use paper towels to wring as much of the water out of the spinach as possible. If the pieces are large, use a sharp knife to roughly chop the spinach. Set aside.
Make the topping. Melt the butter over medium heat in a small skillet. Add the thyme leaves and cook for 30 seconds. Add in the panko and garlic powder and toss to coat in the butter. Cook, stirring occasionally, until golden brown. Season with salt and pepper and set aside.
Saute the shallots and garlic. Melt the butter in a large skillet over medium low heat. Once the butter is melted, add the shallots and garlic. Cook, stirring often, until the shallot is soft but doesn’t have any color on it.
Reduce the cream. Increase the heat to medium. Switch to a whisk, and whisk in the heavy cream, dijon mustard, nutmeg and black pepper and bring to a gentle simmer. Simmer for about 5-6 minutes, until the sauce has thickened a bit. Stir in the chopped spinach and season with salt. Simmer for 5 minutes.
Finish. Remove the pan from the heat and fold in the gruyere cheese. Top with the toasted bread crumbs and serve right away.