Soften the fennel. Heat the olive oil in a large skillet over medium heat. Add the sliced fennel and cook for 3 to 4 minutes, stirring occasionally, until softened and slightly translucent. Add the garlic and stir for another 30 seconds.
Warm the white beans. Add the white beans and cook for another minute, gently stirring in the fennel-garlic oil.
Wilt the spinach. Add the spinach and stir for about a minute, until it's just coated in oil and barely starts to wilt down. Try not to over-wilt it, as it will continue to soften after it's removed from the heat.
Finish the salad. Remove the skillet from the heat and add the lemon juice, salt, black pepper and red pepper flakes (if using). Gently stir in the gorgonzola and walnuts.
Serve. Serve the salad warm and garnish with fennel fronds, if desired.
