Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Clean the portobello mushroom caps with a damp cloth, remove the stems, and carefully scrape out the dark gills using a spoon.
Brush the outside and inside of the mushroom caps with olive oil and season with a pinch of sea salt and black pepper.
Place the salmon fillet and the mushroom caps (gill-side up) on the prepared baking sheet and roast for 12 minutes until the salmon is opaque.
Remove the tray from the oven, flake the cooked salmon into small pieces with a fork, and quickly wilt the baby spinach in a small skillet over medium heat.
Divide the wilted spinach and flaked salmon evenly between the two mushroom caps, pressing down to create a small well in the center.
Carefully crack one egg into the well of each stuffed mushroom cap, ensuring the yolk remains intact.
Sprinkle the eggs with smoked paprika and crumbled feta cheese, then return the tray to the oven for 10-12 minutes until the egg whites are set.
Garnish the stuffed mushrooms with finely chopped fresh chives and serve immediately while hot.