Preheat Panini Press To High.
Heat A Large Skillet Over High Heat.
When Hot, Add Butter, Onions, Mushrooms, Salt, And Black Pepper.
Sauté Until Vegetables Soften, About 3–4 Minutes.
Add Spinach And Continue To Cook Until Excess Moisture Has Evaporated, About 3 More Minutes.
Set Aside.
Slice Baguette On The Diagonal Into Sixteen Even Slices.
Spread Each Piece With ½ Tablespoon Tapenade.
For Each Panini, Layer One Piece Of Bread With A Half Slice Provolone, Then Evenly Divide Tomatoes, Red Peppers, And Sautéed Vegetables On Top.
Top With Another Half Slice Provolone And Another Slice Of Bread.
Grill In Panini Press Until Golden Brown, About 3–4 Minutes.
Serve Immediately.