Ingredients:

  • Olive oil, 1 tablespoon (15 ml)
  • Yellow onion, small, finely diced — 1 (about 90 g)
  • Button or cremini mushrooms, sliced — 8 oz (225 g)
  • Baby spinach, rinsed and drained — 4 cups packed (about 120 g)
  • Salt, 1/4 teaspoon (1.5 g)
  • Black pepper, pinch (to taste)
  • Large eggs — 6 (≈300 g total without shells)
  • Whole milk, 1 cup (240 ml)
  • Heavy cream, 1/2 cup (120 ml) (or 1 1/2 cups / 360 ml half-and-half)
  • Dijon mustard, 1 teaspoon (5 g) — optional
  • Freshly grated nutmeg, pinch (optional)
  • Kosher salt, 3/4 teaspoon (4.5 g) — adjust to taste
  • Freshly ground black pepper, 1/2 teaspoon (to taste)
  • Gruyère or sharp cheddar, grated — 1 cup (100–115 g)
  • Fresh herbs (chives, parsley or thyme), chopped — 2 tablespoons (8 g)
  • Optional: flaky sea salt for finishing

Instructions:

  1. Preheat oven to 350°F (175°C). Lightly grease a 9-inch (23 cm) pie dish or quiche pan.
  2. Heat olive oil in a 10–12 inch skillet over medium heat. Sauté diced onion until translucent and beginning to caramelize, about 5–7 minutes.
  3. Add sliced mushrooms and cook until they release moisture and brown, about 5–7 minutes. Season with 1/4 teaspoon salt and a pinch of black pepper.
  4. Add spinach in batches and cook until just wilted and most moisture has evaporated. Remove skillet from heat and let vegetables cool slightly. Vegetables should be dry to the touch; squeeze out excess moisture if needed.
  5. In a large bowl, whisk eggs until blended. Whisk in milk and cream (or half-and-half), Dijon mustard (if using), a pinch of nutmeg, kosher salt and freshly ground black pepper until smooth.
  6. Scatter half the grated cheese in the bottom of the prepared dish. Spread the sautéed vegetables evenly over the cheese. Pour the egg mixture over the vegetables and gently press them down so they are submerged.
  7. Sprinkle remaining cheese and chopped herbs on top. Bake in the preheated oven for 35–40 minutes, until edges are set and center is slightly wobbly but not liquid; top should be lightly golden.
  8. Test doneness by inserting a knife near the center — it should come out mostly clean with a slight moist crumb. Remove from oven and let rest 10–15 minutes to finish setting.
  9. Slice and serve warm or at room temperature. To store: cool to room temperature, cover and refrigerate up to 3–4 days. Reheat gently in a 325°F (160°C) oven or microwave slices.