Boil fresh spinach in salted water for about 2-3 minutes, until it wilts. Drain. Set aside.
In a deep pan, add oil and garlic, and fry for 30 seconds. The garlic should not darken. Then add flour and stir with a whisk for 20 seconds. Add milk and mix well. If it is too thick, add a bit more milk.
Add drained spinach and mix well. Simmer for 2 minutes while stirring. Remove from the heat.
Add butter and egg yolk, and mix well. Add salt and black pepper, then return to the hob for a few moments to heat everything through once more.
Notes
The total estimated calories for the whole dish is around 680. This dish serves 2 people. Please remember that the calorie counts are approximate and may vary based on specific products and cooking methods used.