Roast foil wrapped beets on a baking sheet, in a 400° oven for 50 to 75 mins, depending on size, until a paring knife easily penetrates with little resistance. Set aside to cool or refrigerate for 1 day.
Remove the skin and root end and dice the beet into 1/4 inch or less pieces, place in a bowl.
In another bowl, large enough for the beets, mix the onion, horseradish, oil, lemon juice, capers, parsley, salt and pepper, then fold in the beets.
Mix the sour cream and blue cheese in a small bowl.
Brush both sides of the baguette slices lightly with olive oil and toast in a hot skillet, 1 or 2 minutes per side.
To serve, spread the baguette with cheese and add the beet tartare, garnish if desired.