Preheat oven to 350 degrees.
Heat olive oil in a nonstick skillet over medium heat. Add onion and sauté 2 minutes. Add garlic and sauté 1 more minute.
Add spinach and sauté 30 seconds. Turn heat off and stir to wilt spinach evenly. Season with salt and pepper. Let cool and then press paper towels down on top to remove excess moisture.
Place chopped artichoke hearts in a medium bowl. Add mascarpone and mayonnaise. Stir to combine.
Add onion/spinach mixture to bowl along with both cheeses, red pepper flakes, thyme, paprika, and dry mustard. Fold everything together.
Spoon into a 1-quart baking dish, sprinkle with Panko crumbs, and bake for 20 minutes.
Combine butter and olive oil in a small microwave-safe bowl and microwave until butter is melted.
Brush both sides of bread slices with butter mixture, place on a baking sheet, and bake 8 to 10 minutes, until lightly toasted.
Serve dip warm with bread slices.