Rinse the baby spinach thoroughly in cold water to remove grit. Spin or pat dry with towels until mostly dry. Peel and mince the garlic cloves. Peel the shallot and finely mince (about 2 tablespoons). Measure out oil, butter, salt, pepper, lemon juice, red pepper flakes, and Parmesan if using.

Heat pan

Place a large skillet or sauté pan over medium heat. Add the extra-virgin olive oil and the butter. Heat until the butter melts and the mixture shimmers but is not smoking (about 1 minute).

Sweat aromatics

Add the minced shallot to the pan and cook, stirring, for 1 to 1 1/2 minutes until softened and translucent. Add the minced garlic and red pepper flakes and cook for another 20–30 seconds until fragrant, taking care not to brown the garlic.

Add spinach in batches

Increase heat to medium-high. Add about half of the spinach to the pan and use tongs to toss and press it down so the leaves start to wilt. After 30–45 seconds, add the remaining spinach and continue to toss and press until all leaves are just wilted but still bright green, about 1 to 2 minutes total. Work quickly to avoid overcooking.

Season and finish

Sprinkle the kosher salt and black pepper over the wilted spinach. Pour the lemon juice over and toss to combine. Remove the pan from heat. If using, sprinkle the grated Parmesan over the spinach and give a final toss so it melts slightly into the warm leaves.

Serve

Transfer the warm wilted spinach to a serving bowl or plates. Taste and adjust seasoning with additional salt, pepper, or lemon juice if needed. Serve immediately as a side or warm topping.